The home secretary is going to make an urgent statement in the Commons on Olympics security, after it emerged an extra 3,500 troops were needed.
The defence secretary had been due to submit a written statement but Labour tabled an urgent question to the House.
The troops are in addition to 13,500 already agreed, amid fears contractor G4S may not have enough trained staff.
G4S said it had "encountered some delays" in processing applicants through the final stages of training.
Labour's Tessa Jowell said there was "clearly a serious problem".
Meanwhile, the chief inspector of borders has raised fresh concerns about the border controls at Heathrow in the run-up to the Olympics.
G4S is being paid £300m supply 10,000 guards for the Games, but the BBC understands it has not been able to guarantee it can deliver that number.
Security minister James Brokenshire told BBC Radio 4's Today programme that G4S had previously been very clear about their ability to comply with the terms of their contract.
"We've been challenging them, asking the questions, really going down and kicking the tyres and doing all those sorts of things and they have been very robust about the fact that they would deliver on the contract. It has only been in these last few weeks that these issues have emerged."
He said the agreed fee to G4S included payments to guards and that it would obviously be reduced if the company provided fewer of them.
In a statement, G4S said its Games deployment was "unprecedented and very complex" and being "carried out to a very tight schedule".
The company said it had 4,000 staff working across 100 venues and more than 9,000 still being trained, vetted and accredited.
"We have encountered some delays in progressing applicants through the final stages but we are working extremely hard to process these as swiftly as possible," it said.
"We understand the government's decision to bring in additional resources and will work with Locog, the military and other agencies to deliver a safe and secure Games."
'Safe and secure'
In a statement, the Home Office said: "We have agreed to offer help to G4S by revising the level of military support."
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"The government is committing £553m for venue security and we remain confident that we will deliver within budget."
Keith Vaz, the Labour chairman of the Home Affairs Select Committee, said he would write to the home secretary for an explanation.
He said he was "deeply concerned" that G4S were unable to deliver their contract and that "the Home Office have left contingency plans to the last moment".
Mr Vaz later said he had asked G4S's chief executive and chairman to appear before the committee next week to explain why they did not have sufficient staff for the Olympics.
Ms Jowell said it was "imperative" that action was taken to ensure there were adequate staff to make the Games safe and secure.
Tessa Jowell: "It's a matter of concern that this failure of G4S seems to have emerged"
"We need to know why the problem has emerged so late in the day and precisely what has been agreed to," the shadow Olympics minister said.
"We also must know whether this affects Army commitments elsewhere, which units are providing people and what terms and conditions are given for those who will likely lose periods of leave."
Retired Colonel Richard Kemp, a former infantry commander in the British Army, said the timing of the announcement was "bizarre".
"We've known about the Olympic Games for seven years, if we couldn't have planned better than this then there's something wrong," he told BBC Radio 4's Today programme.
Colonel Kemp said he understood many of the extra troops being drafted in had just returned from Afghanistan - or were about to be deployed there - and would have been due to go on leave.
Sources at the Ministry of Defence said the extra military personnel would be compensated if they had their leave cancelled.
The BBC understands the extra troops were put on a "one day" notice to move recently, in anticipation of the problems at G4S.
The government has previously said a total security force of 23,700 will take care of venue security, including 13,500 troops and the G4S contingent.
G4S will have overall control of security at more than 30 Olympics sporting venues and 70 "non-competition" venues such as car parks and hotels.
Some of the people contracted to work for G4S have told the BBC the company has given them little information.
One caller to BBC Radio 5live, Jane, from Manchester, said she had heard nothing from G4S since April.
"I had an interview in March and I had training for a security licence in April, and then since then there has been very little information about what actually the job role is," she said.
"I have had no job training. I don't even know where I'm going to work, or what kind of work its going to be or when it's going to start."