London 2012: G4S boss faces questions from MPs
The G4S chief executive will go before a committee of MPs later to explain why his firm were unable to provide enough Olympics security staff.Nick Buckles has already apologised after 3,500 extra troops had to be deployed to meet the firm's shortfall.
Culture Secretary Jeremy Hunt said it was no time for a "witch hunt" but "contingency plans" were in place if G4S further failed to deliver.
Theresa May said G4S had "repeatedly" promised they would exceed targets.
Mr Buckles, who is due to appear before the Commons Home Affairs select committee, has said he is "bitterly disappointed" at his company's failure to meet the terms of its contract.
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London 2012 - One extraordinary year
The company by its own admission stands to lose up to £50m on the contract, worth a total of about £280m, after being unable to provide the 10,000 staff it had been contracted to deliver.The culture secretary told the BBC's Newsnight programme: "We would have been failing in our job as ministers if a contract had gone wrong and we didn't have a back-up plan that worked."
He said they would continue to monitor the contract.
Asked if more troops might be neccessary, he replied: "Of course if G4S don't deliver what they now say they can provide we have contingency plans."
But a Ministry of Defence spokesman told the BBC: "Olympic security remains a civilian and police-led operation which has not changed.
"For many months the MoD has been working closely with G4S with military personnel embedded as Olympic security plans have developed. As you would expect, the level of liaison has increased as the Games has drawn closer and the military contribution has increased."
Police meanwhile have had to deploy extra officers from eight UK forces to do Olympic security work after G4S staff failed to turn up.
Greater Manchester Police had to deploy officers to provide security at a hotel in Salford where four Olympic football teams will stay - after only 17 of an expected 56 G4S staff turned up for work.
In the Commons on Monday the home secretary reiterated the government only knew on Wednesday that there would not be enough G4S security guards.
In her statement to MPs, Mrs May insisted the government had reacted quickly when it discovered G4S was in difficulty.
"G4S only told the government that they would be unable to meet their contractual arrangements last Wednesday and we took immediate action," she said.
She also denied G4S had "deliberately deceived" the government, insisting the firm's problem was "workforce supply and scheduling".
The prime minister's official spokesman refused to be drawn on when David Cameron was aware of G4S's Olympic recruitment failings, but said he was involved in the decision to deploy extra soldiers.