The military will have to provide an extra 3,500 troops for security during the Olympics, after private security firm G4S admitted it may not be able to supply all of the guards needed.
It means that the 23,700 strong security force for the Games will feature some 17,000 troops, in addition to private security guards and unpaid London 2012 volunteers.
The G4S workforce had been contracted to carry out physical searches, perimeter searches, and operate X-ray machines at the Games.
A G4S spokeswoman said:
Shadow Olympics minister Dame Tessa Jowell said the last-minute changes were a "serious problem" and called for the Government to explain whether it would affect Army commitments elsewhere and "what terms and conditions are given for those who will likely lose periods of leave".
There are also concerns that the move could seriously dent morale at what is already a tough time for the military. The Army is facing its biggest re-structuring in decades, with 17 major units set to be axed.
A Home Office spokeswoman said: "We have agreed to offer help to G4S by revising the level of military support.
"The Government are committing £553 million for venue security and we remain confident that we will deliver within budget."
She added: "Our approach is intelligence-led and risk-based, and we retain the ability to be flexible in our response."
A London 2012 spokesman added: "Security for the Games is big and complex but we have the best brains in the security business working on this - Home Office, Metropolitan Police, Ministry of Defence and world's largest private security business.
"Delivering a safe and secure Games is everyone's number one priority.
"We do not anticipate an increase in the overall venue security numbers."