An inquiry into the death of Russian spy Alexander Litvinenko has been asked to consider whether personal attacks against President Vladimir Putin and attempts to expose "corruption" in the Federal Security Service (FSB) resulted in him being poisoned.

Speaking nine years after the former Russian secret service agent was poisoned with highly radioactive Polonium-210 in London, chairman of the public inquiry into Litvinenko's death, Sir Robert Owen, said his killing raises issues of the "uttermost gravity" which have attracted "worldwide interest and concern".

Litvinenko, 43, died in November 2006 three weeks after ingesting the poison. It is alleged that his tea had been laced with polonium when he met two former Russian colleagues, Andrei Lugovoi and Dmitry Kovtun, at the Millennium Hotel in Mayfair.

Both Lugovoi and Kovtun deny responsibility for Litvinenko's death. Russia has also refused to extradite the pair so they could face possible prosecution.

Owen has previously said there is British government evidence which establishes a "prima facie" case showing the Russian state is responsible for the spy's death.

The inquiry heard how after fleeing to the UK, Litvinenko became a fierce critic of the Kremlin while allegedly working for MI6.

Counsel to the inquiry, Robin Tam QC, said this included writing a series of books and articles claiming there was what he called an "endemic" of corruption in the FSB, as well as personal attacks against Putin.

In one unsubstantiated article, published on a Chechnya website, Litvinenko accused Putin of being a paedophile after he kissed a five-year-old boy on the stomach after stopping to speak to tourists on Kremlin grounds.

The incident was covered and dismissed as merely innocent by the world's press in 2006. However, Litvinenko wrote: "The world public is shocked. Nobody can understand why the Russian president did such a strange thing as kissing the stomach of an unfamiliar small boy.

"The explanation may be found if we look carefully at the so-called 'blank spots' in Putin's biography."

The court also heard that after his house was firebombed in 2004, Litvinenko publicly blamed Putin for the attack during an event at London's Frontline Club, as well as the murder of Russian journalist Anna Politkovskaya. The reporter - who was known for her opposition to Putin - was killed in October 2006.

On his deathbed, when asked who he thought was responsible for ordering his poisoning, Litvinenko said: "That person is the President of the Russian Federation, Vladimir Putin."

Tam told Owen: "You will need to consider whether Litvinenko's sustained public attacks on the regime, on the FSB and on Mr Putin in particular, could have had any connection with his death."

He added: "At the extreme end of the spectrum in July 2006, Mr Litvinenko published an article on the Chechen press website accusing Putin of being a paedophile.

"Could the Kremlin have regarded him as an irritant or worse?"

Prime suspects Lugovoi and Kovtun had been invited to give evidence to the inquiry via video link from Russia - an invitation Owen hopes "will be accepted".