An extra 3,500 armed troops will be drafted in to guard the 2012 Olympics at the last minute, due to concerns that private security firm G4S may not have sufficient trained manpower.

The Ministry of Defence (MoD) has already committed 13,500 troops to guard the Games' 30 sporting and 70 non-competition venues. Defence secretary Philip Hammond is expected to announce a significant increase to this figure imminently.

G4S is obligated to provide 13,700 guards under the terms of its Olympics contract. However, only around 4,000 guards are ready to be deployed at present.

The contractor claims that a further 20,000 guards are currently being trained, vetted and accredited. However, it has admitted that "we have encountered some issues in relation to workforce supply and scheduling over the last couple of weeks."

With just two weeks to go until the opening ceremony, the last-minute move to deploy extra troops has drawn sharp criticism from the opposition.

Keith Vas, Labour MP and chairman of the Commons Home Affairs Select Committee, urged home secretary Theresa May to explain the issue as a matter of urgency.

"With only 15 days to go until the Olympics, we now find out that she [Ms May] has apparently called on the military to cover a shortfall of over 3,500 security guards. I am deeply concerned that G4S are now unable to deliver their £284m contract and that the home office have left contingency plans to the last moment," Vaz told Sky News.

Shadow Olympics minister Dame Tessa Jowell also criticised the move, saying the contingency measures demonstrated "serious problems" and calling for an explanation of how the extra Olympics burden would impact the army's other commitments.

London Theresa May