The military is to provide up to 3,500 extra troops for security duty at the Olympics amid fears that private firm G4S will not be able to supply enough personnel.
The move, just a fortnight before the Games, is potentially a major embarrassment for organisers Locog.
The overall 23,700-strong security force for the Games will include a mix of military, private security guards and at least 3,000 unpaid London 2012 volunteers who will be used at the start of the security process.
Video report by Keir Simmons
A G4S spokeswoman said: "This has been an unprecedented and very complex security recruitment, training and deployment exercise which has been carried out to a tight timescale.
"We have encountered some issues in relation to workforce supply and scheduling over the last couple of weeks, but are resolving these every day and remain committed to providing a security workforce for the start of the London 2012 Games."
She went on: "Our planning with Locog and other security agencies allows for a variety of contingencies which have been reviewed in the build-up to the Games.
"We accept that the Government has decided to overlay additional resources.
"We remain committed to keep London 2012 safe and secure."
Home Secretary Theresa May was forced to defend G4S in the Commons on Monday, saying she was "confident our partners will deliver a safe and secure Games".
More than 100 different venues need to be protected during the Games, she told MPs.
13,500 troops already involved
Some 13,500 troops are already involved in a wide variety of Olympics-related roles across the UK, including 7,500 for venue security.
Today's move will boost the number of servicemen and women involved in venue security to 11,000 and the overall number of troops involved in the Olympics to some 17,000, it is understood.
Along with venue security, military personnel will also be involved in specialist support roles including air security, search teams, communications and logistics, among others.