The United States is preparing to send 1,000 security agents, including 500 from the FBI, to protect its athletes and diplomats at the London Olympic Games amid concerns that the UK has not prepared for the event with adequate care, it has been reported.
According to the Guardian, the police response to the summer riots undermined US officials' confidence in the UK's ability to stage the event.
There are also concerns about recent restrictions placed upon antiterrorism 'stop and search' measures.
US officials have now requested detailed plans about the size and nature of the UK's policing plans, and have pushed aggressively for a larger presence of its own armed security teams in London.
The paper quoted one Whitehall source who said every element of the plan had been prepared "with the US in mind".
"If something happens and we say we did not have enough people we are finished," the Guardian's source reportedly said.
British officials are said to be "exasperated" by the level of US meddling in their plans, the newspaper said.
Prime Minister David Cameron, Home Secretary Theresa May and the culture secretary, Jeremy Hunt, have all chaired security meetings about the Olympics that have been "dominated" by the latest questions from the US.
The London Organising Committee of the Olympic and Paralympic Games (Locog) is also under pressure after it under-estimated the number of security personnel that would be required at the 32 venues.
Originally Locog estimated that around 10,000 security guards would be needed, but it has now determined more than twice that number will be required.
To make up that shortfall Locog has asked the private firm charged with venue security, G4S, to hire and train additional staff. However the committee does not believe the company will be able to make up the numbers in time, and has turned to the Ministry of Defence for help.
The Guardian says that the MoD has offered around 3,000 troops with another 2,000 in reserve, which is around half of the additional total needed.
"There is less than a year to go and they've only just realised they need twice the number of security guards they first thought." The Guardian quoted one official as saying. "Where is the money to pay for this coming from? It is an extra burden on the defence budget that we could well do without."
The SAS recently denied that it was building a secret underground Olympics HQ to enable a quick response to a Mumbai-style attack on the games.
It has also been reported that a fleet of high-powered military inflatables will be kept on standby on the East London riverside to dispatch special forces and that a purpose-built SAS security centre will also form part of a £600 million security plan.
A Home Office spokesman said: "Security planning is on track and funding has been protected. The Government is committed to delivering a safe and secure Games that London, the UK and the world can enjoy.
"The International Olympic Committee (IOC) undertake detailed inspections of security preparations and have full confidence in our plans. The UK has a strong and close working relationship with the US, who have expressed similar confidence.
"The Government, London 2012 Organising Committee (LOCOG) and G4S are working together to finalise the requirement for venue security and as is common at major events in the UK we will make the best and most appropriate use of all available resources."
The US state department declined to comment, and the FBI was not available for comment.