The US reportedly wants to send its own agents to protect athletes and diplomats at next year's Olympics - but the Government insists security plans are "on track".
America is preparing to send up to 1,000 agents - including 500 from the FBI - across the Atlantic next summer, according to The Guardian.
The newspaper reported that US officials have raised "repeated concerns" about security and are worried the UK has had to restrict the scope of anti-terrorism "stop and search" powers.
They are also reported to be anxious about the police response to the London riots .
Earlier this year, National Olympic security co-ordinator Chris Allison, of the Metropolitan Police, said he believed 12,000 officers will be needed nationally to police the event.
Another 10,000 to 15,000 security officials could also be deployed by firm G4S.
There are also plans to use large numbers of military personnel to police the games - including SAS troops stationed in boats along the Thames in case of a Mumbai-style terror attack.
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 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 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."
Sky News' security editor Sam Kiley said: "Britain is a significant terrorist target... from the American perspective, you can't be too careful."
A Home Office source said it did "not recognise as true" suggestions that there are concerns from the US.
The US state department declined to comment, and the FBI was not available for comment.