An inquiry into the poisoning of Alexander Litvinenko has found that the former Russian spy's assassination was "probably" approved by Russian President Vladimir Putin.

The report, which could have far-reaching repercussions in diplomatic relations between the UK and Russia, also concluded that the two main suspects in the case, Andrei Lugovoi and Dmitry Kovtun, "probably" poisoned Litvinenko by lacing his tea with radioactive polonium-210 at a London hotel.

Presenting the report, inquiry chairman Sir Robert Owen said he is "sure" the two men "knew that they were using a deadly poison and that they intended to kill Mr Litvinenko".

Statement by the chairman of the Litvinenko inquiry, Sir Robert Owen

Alexander Litvinenko was born on 4 December 1962, a citizen of the Soviet Union. He died aged 44 on 23 November 2006 in University College Hospital London, by then a British citizen.

Post-mortem examination revealed that his death had been caused by an ingestion of a fatal dose of the radioactive isotope polonium-210. The circumstances of his death attracted worldwide interest and concern.

They were referred to by the foreign affairs select committee as "... a miniature nuclear attack on the streets of London". In July 2007 the then Foreign Secretary observed that "the manner of Litvinenko's death put many hundreds of other people at risk".

A motion of the United States House of Representatives dated 1 April 2008 noted that polonium-210 "could be used to kill large numbers of people, or spread general panic and hysteria among the public".

In the course of the inquest hearings, it was submitted on behalf of media organisations that the issues to which it gave rise "... include allegations of state-sponsored assassination by radioactive poisoning ..." of a British citizen in London, issues of the greatest public concern.

Over nine years have elapsed since his death; and it is appropriate that I should explain shortly why it has taken so long for a full enquiry into his death to be completed. The inquest into his death was opened by Her Majesty's Coroner for Inner North London on 30 November 2006 but was adjourned pending the police investigation into his death and any ensuing criminal proceedings.

The police investigation led to the conclusion that the fatal dose of polonium-210 was probably consumed by Mr Litvinenko on 1 November 2006 when in the company of Mr Andrei Lugovoy and Mr Dmitri Kovtun, Russian nationals, at a hotel in London.

Warrants were in due course issued for their arrest, and the Crown Prosecution Service sought unsuccessfully to extradite them from the Russian Federation to stand trial for murder. On 13 October 2011, the inquest was resumed as it had become clear by then that there was no realistic prospect of the suspects facing a criminal trial, and on 7 August 2012 I was appointed to conduct the inquest.

In the course of my preparation for the inquest, I was given access to sensitive government documents that in my judgment were relevant to the investigation that I was conducting. More particularly, those documents raised a prima facie case that the Russian state bore responsibility for Mr Litvinenko's death.

The law does not permit evidence to be taken in what are known as secret or closed sessions at an inquest. But the government material was of such sensitivity that it could not be produced in any form of public or open hearing.

The material was therefore necessarily excluded from the inquest proceedings under the legal principle known as public interest immunity. It had always been my view that the question of possible Russian state responsibility was one of the most important issues arising from his death.

It was an issue that I had wanted to investigate at the inquest, but I considered that I would be failing in my duty to conduct a full and independent investigation if I did so in the knowledge that there was relevant government material that I could not take into account because of public interest immunity.

I therefore wrote to Her Majesty's Government asking it to exercise the power to establish a public inquiry to replace the inquest. I did so because under section 1(1) of the Inquiries Act 2005, that power may be exercised where it appears to a minister that: "Particular events have caused or are capable of causing public concern or there is public concern that particular events may have occurred."

The advantage of a public inquiry over an inquest was that the rules governing an inquiry allow for sensitive evidence to be heard in closed session. However, the Home Secretary declined my request. But her refusal to establish a public inquiry was successfully challenged in the High Court by Mr Litvinenko's widow, Marina Litvinenko.

The judgment in which the divisional court upheld the challenge was handed down on 11 February 2014. It required the Home Secretary to make a further decision as to whether to establish an inquiry.

Thus, on 22 July 2014, almost two years after I had been appointed to conduct the inquest, the Home Secretary announced in a written statement laid before the House of Commons that a public inquiry was to be held into the death of Alexander Litvinenko under the Inquiries Act 2005, and in consequence the inquest was suspended. I was appointed to chair the Inquiry.

I was then a serving judge of the High Court, an office from which I retired on 19 September 2014, having reached the compulsory retirement age. But my retirement did not affect my position as chairman of the Inquiry. The terms of reference for the Inquiry, terms upon which I was consulted, are set out in full in my report and on the Inquiry website.

Paragraph 1 provides as follows: 1. Subject to paragraphs 2 and 3 below, the chairman is to conduct an investigation into the death of Alexander Litvinenko in order to: (i) ascertain, in accordance with section 5(1) of the Coroners and Justice Act 2009, who the deceased was; how, when and where he came by his death; and the particulars required by the Births and Deaths Registration Act 1953 to be registered concerning his death; (ii) identify, so far as is consistent with section 2 of the Inquiries Act, where responsibility for the death lies; (iii) make such recommendations as may be appropriate. By her letter of appointment, the Home Secretary invited me to complete the Inquiry by December 2015.

I opened the Inquiry on 31 July 2014. The final open hearing took place a year to the day later on 31 July 2015. On 15 December last, I announced that my report was complete and that in accordance with the protocol agreed with the Secretary of State, the report would be delivered to her 48 hours before being tabled by her in the House of Commons at 9.35 today.

It will shortly be published on the Inquiry website. The Inquiry has been completed well within the budget prepared by the Inquiry secretariat and adopted by the Home Secretary in setting a budget cap. I conducted open hearings at the Royal Courts of Justice on 34 days in January, February, March and July 2015.

The open evidence is available to the public in its entirety on the Inquiry website, the oral evidence in the form of full daily transcripts. The witness statements and the documents admitted into evidence are also available on the website. I also held closed hearings in the course of which I heard oral evidence and considered documentary material, the subject of restriction notices. The findings of fact and the conclusions that I have drawn from the facts are based upon the entirety of the evidence that I have seen and heard, both open and closed.

They are mine and mine alone. I turn then shortly to summarise the central findings of fact and my conclusions as to how, when and where Alexander Litvinenko came by his death, and as to where responsibility for that death lies. Alexander Litvinenko was born on 4 December 1962 in the Russian city of Voronezh. He attended military college graduating in about 1985 as a lieutenant and served for approximately three years in the forces of the interior ministry.

In 1988, he was recruited to join what was then still called the KGB, and in 1991 was posted to KGB headquarters in Moscow. In September 2000, he left Russia in the circumstances that I consider in detail in part 3 of my report. On 1 November 2000, he arrived in London with his wife Marina and his son Anatoly where, in the transit area of London airport, he approached the first police officer that he saw and said, "I am a KGB officer and I'm asking for political asylum".

He was granted asylum in due course, and on 13 October 2006, he and his family were granted British citizenship. 41 days later, Alexander Litvinenko died at University College Hospital, London. The immediate cause of death was a cardiac arrest from which the medical staff at the hospital were unable to resuscitate him. But the cardiac arrest was the result of an acute radiation syndrome caused by his having ingested approximately 4.4 gigabecquerel of polonium-210.

The evidence indicates that there was more than one intake of polonium-210. The second, and fatal, ingestion occurred on 1 November 2006, the sixth anniversary of his arrival in the United Kingdom seeking asylum. The first, which had been about a hundredth of the size, had occurred some 14 days earlier.

The evidence establishes that Mr Litvinenko ingested the fatal dose whilst drinking tea from a teapot contaminated with polonium-210 in the Pine Bar of the Millennium Hotel in the West End of London in the afternoon of 1 November in the company of Andrei Lugovoy and Dmitri Kovtun. I am sure that Mr Lugovoy and Mr Kovtun placed the polonium-210 into the teapot at the Pine Bar and did so with the intention of poisoning Mr Litvinenko.

I am also sure that the two men made the earlier attempt to poison Mr Litvinenko, also using polonium-210, at a meeting on 16 October 2006. I am sure that Mr Lugovoy and Mr Kovtun knew that they were using a deadly poison and that they intended to kill Mr Litvinenko. I do not, however, believe that they knew precisely what the chemical that they were handling was or the nature of all its properties.

Mr Litvinenko did not, as has been suggested by Mr Lugovoy and those representing him in the early stages of the inquest proceedings and by other commentators, poison himself with polonium-210, either accidentally or deliberately. The scientific evidence as to the sites of primary contamination by polonium-210 detailed in part 6 of my report demonstrates conclusively that Alexander Litvinenko was poisoned by Andrei Lugovoy and Dmitri Kovtun.

The further suggestion that has been made that Mr Lugovoy had been the subject of a "set-up" is simply unsustainable by reference to the objective scientific evidence. There can be no doubt that Alexander Litvinenko was poisoned by Mr Lugovoy and Mr Kovtun. The open evidence upon which I have arrived at that conclusion is set out in considerable detail in parts 4, 5, 6 and 8 of the report. My finding that Mr Litvinenko was poisoned by Mr Lugovoy and Mr Kovtun raises a further question.

There is no evidence to suggest that either had any personal reason to kill Mr Litvinenko. All the evidence points in one direction, namely that when they killed Mr Litvinenko, they were acting on behalf of someone else.

I have concluded that there is a strong probability that when Mr Lugovoy poisoned Mr Litvinenko, he did so under the direction of the FSB, the Federal Security Service of the Russian Federation. I have further concluded that Mr Kovtun was also acting under FSB direction, possibly indirectly through Mr Lugovoy, but probably in the knowledge that that was the body for which he was acting.

I have further concluded that the FSB operation to kill Mr Litvinenko was probably approved by Mr Patrushev, then head of the FSB, and also by President Putin. These findings relating to Russian state responsibility are based on evidence which I heard in open and closed sessions of the Inquiry.

My reasoning for these conclusions in open is to be found in parts 9 and 10 of the report. That concludes my short summary of the Inquiry's core findings. I should note at this point that, despite the welter of public comment and speculation since November 2006, there has not, until now, been a formal and independent investigation tasked with enquiring into and making findings about the circumstances of Mr Litvinenko's death. Our law requires that all violent or unnatural deaths are investigated in this way.

Had Mr Lugovoy or Mr Kovtun been put on trial in this country for Mr Litvinenko's murder, those proceedings may well have satisfied this requirement. But, for the reasons that I have explained, there have been no such criminal proceedings. The requirement for a formal investigation into Mr Litvinenko's death was therefore outstanding.

With the additional advantage, over and above the evidence that an inquest or criminal trial would have been able to consider, of having been able to examine sensitive evidence in the closed sessions, that is the function that I have now fulfilled. As I have explained, the inquest into the death of Alexander Litvinenko was suspended when my Inquiry was established.

I am not currently minded to reopen the inquest as I have addressed all of the matters that I would have been obliged to consider as a coroner. Should any of the core participants wish for any reason to apply to me to reopen the inquest, they should communicate with the solicitor to the Inquiry within 28 days, and I will make a decision as to whether there is sufficient reason to do so under paragraph 9(1) of the schedule of the Coroners and Justice Act 2009.

It remains only for me to express my thanks to all who have assisted me in this Inquiry, counsel and solicitor to the Inquiry, the Inquiry secretariat, the counsel and solicitors acting for the core participants, in particular counsel and solicitor acting for Marina and Anatoly Litvinenko, to the Metropolitan Police who carried out the original criminal investigation into the death of Alexander Litvinenko, an investigation exemplary in its breadth and in its depth, and to all of those who have facilitated the efficient running of the proceedings, in particular the teams responsible for the technical aspects of the Inquiry hearing, including the simultaneous transcription of the evidence, and the team of ushers who assisted greatly in the management of the hearings.