Theresa May was tonight accused of giving MPs a "selective account" about when she knew G4S were having problems supplying enough guards to secure the Olympics.
The Home Secretary admitted the Government and organisers Locog knew there were problems and that the firm, the world's second largest private sector employer, could fail to meet its contract as early as June 27.
It comes after the company's under-pressure chief executive Nick Buckles told MPs that he did not tell organiser Locog about the problems until a week later and Mrs May told the Commons the "absolute gap in numbers" was not known until July 11.
The revelation came as a further 1,200 troops were put on standby to provide Olympics security tonight as the fallout from the shambles continued.
But the Government said the numbers of staff provided by the company were rising, and there was currently no need to deploy more military personnel.
Shadow home secretary Yvette Cooper said: "The Home Secretary was asked repeatedly when she and the Home Office were warned about problems at G4S, and she repeatedly gave everyone the impression they had only known since July 11.
"Why has Theresa May waited until now to admit the Home Office in fact were warned two weeks earlier?
"She needs to explain urgently how she justifies having given Parliament and the public such a selective account, and why the Home Office were so slow to respond."
The details were revealed by Mrs May in a letter to the Commons Home Affairs Select Committee.
He called for any updates on the figures contained in monthly internal assurance reports to be released and for an explanation as to "why they did not ring alarm bells sooner".
But a Home Office spokesman said: "Mr Buckles' evidence to the Home Affairs Select Committee this week confirmed what the Home Secretary told the House of Commons: G4S did not tell ministers that they would be unable to deliver their contractual obligations until July 11."
While 750 troops were put on 24 hours' notice last month, the extra 3,500 servicemen and women who will plug the gap left by G4S were not called in until two weeks' later after the company confirmed its "absolute" shortfall.
Mrs May admitted that permission to put a small part of the military contingency force on 24 hours notice was sought as early as June 28, with the force being mobilised "several days later".
This was increased to 725 troops by July 9 and plans to put even more troops on standby got under way on July 6, she added.
"But at this stage, of course, G4S were still confident that they would deliver the required numbers," Mrs May said.
"However, as we now know, this is no longer the case.
"On July 11 G4S told the Olympic Security Board for the first time that they were no longer confident of reaching their workforce targets."
Some 3,500 servicemen and women were brought in to boost the number of military personnel involved in Olympics security to 17,000, including 11,000 to secure Games venues.
Mrs May told the Commons last week that "the absolute gap in numbers was crystallised" on Wednesday July 11, but that talks with the Ministry of Defence (MoD) had taken place earlier to ensure troops would be available if needed.
Culture Secretary Jeremy Hunt said an extra 1,200 troops were being put on 48 hours' notice to move so they could be called upon if needed in the coming weeks.
"We hope that will not be necessary but this is a sensible precaution," he said.
"Ministers agreed today that there is no current need to deploy any additional troops.
"G4S numbers are rising and we are seeing an improvement in the company's performance, which is to be welcomed.
"There will be other challenges over the coming weeks, but we are confident that we are on track to stage a great Games."
Earlier, Ed Miliband called for G4S to be blocked from getting new Government contracts in the wake of the Olympics security shambles.
But he refused to call for Mr Buckles' immediate resignation, suggesting the Games needed stability.
In a statement tonight, a G4S spokesman said the firm "believes that, even if some or all of the additional troops now on standby were to be deployed, the overall losses to be incurred on this contract would remain within the previously stated estimate of £35-£50 million."
He went on: "The company also wishes to clarify the nature of the £57 million 'management fee' referred to at the Home Affairs Select Committee on Tuesday July 17.
"This constitutes payments in respect of the operating cost elements of the contract relating to the infrastructure needed to support the contract over the last two years.
"During that time G4S has incurred significant costs on behalf of its customer, Locog, to provide it with a range of staff, facilities and services including premises, IT and payroll systems."
The firm's spokesman went on: "G4S continues to focus huge resource and effort on increasing the number of G4S personnel engaged in the Olympics security programme.
"We are making good progress, with daily increases in the number of people working at Olympics venues as well as in the number of people who have completed the necessary training and related accreditation processes."
He added the firm recognised that putting 1,200 extra troops on standby was "a sensible precaution".
"We are working very closely with Locog, the military, police and Government and we expect to continue to build on the progress we have made in the past few days," he said.
"G4S has undertaken to cover the additional costs relating to the military and police who make good any shortfall in G4S personnel."