British jihadis are reportedly plotting to assassinate the Queen at an event to mark the 70th anniversary of VJ Day – Victory in Japan and the end of the Second World War.
According to The Mail on Sunday, police and MI5 have received intelligence suggesting the assassination plot is being orchestrated from Syria by Islamic State commanders.
The extremist plot is believed to involve explosions using deadly pressure cooker bombs with the intention of causing a high numbers of casualties, as thousands of members of the public are expected to line the streets around Whitehall and Westminster Abbey.
The Mail Online reports that sources have told the publication that a "specific threat was made against the Queen", prompting an urgent review of security arrangements by Scotland Yard.
Senior members of Scotland Yard's Royal Protection branch, SO14, have been informed of the threat, along with SO1 Specialist Protection, the unit responsible for the security of the Prime Minister and any dignitaries considered to be 'under threat from terrorist attack'. The Home Office has also been made aware of the terror plot.
Prince Charles, who is expected to attend the weekend celebrations, is also identified in intelligence reports as another possible target.
The senior royals will join Prime Minister David Cameron, dignitaries, more than 1,000 veterans of the Far East campaign and their families, and serving members of the Armed Forces.
A similar device was used in the 2013 Boston Marathon bombing which killed three people and injured more than 260.
In the July 7 London bombings, 52 people were killed and more than 700 injured when four suicide bombers struck on Tube trains and a bus in Central London in 2005.
The weekend's events, which will be broadcast live on television by the BBC, will see the Queen, accompanied by the Duke of Edinburgh, attend a service at St Martin-in-the-Fields in Trafalgar Square. Dignitaries will move to Horse Guards Parade for a traditional military 'drumhead' service, a flypast and a wreath-laying ceremony before the set-piece VJ Day parade of veterans and current troops down Whitehall to Westminster Abbey.
Fears of a terror attack on UK soil were heightened after the Tunisian beach massacre, in which 38 tourists died, including up to 30 Britons, with Prime Minister David Cameron expressing his concerns of a 'serious threat' of an attack on home soil.
"There are people in Iraq and Syria who are plotting to carry out terrible acts in Britain and elsewhere and as long as ISIL (Islamic State) exists in those two countries we are at threat," Cameron told BBC radio on Monday 29 June. He also said that the fight against IS is "the struggle of our generation".
Two months ago, a series of terror manuals providing step-by-step instructions on how to launch 'lone wolf' attacks on the UK and the West were circulated online by Isis.
Last November, police thwarted an alleged plan to launch an attack on Remembrance Sunday after arresting four men in London and High Wycombe, Buckinghamshire.
The Mail Online reports that the Queen has been warned of the terror threat, but has indicated her determination to attend the event to "honour the fallen".
A Buckingham Palace spokesman said of the terror plot: "It is not something we would comment on" while a Scotland Yard spokesman declined to comment.
No arrests have been made so far in connection with the alleged plot, however counter-terrorism officers are said to be monitoring the threat.