The Home Office was warned of a "possible temporary shortfall" in G4S guards for the Olympics as early as June 27, the Home Secretary has admitted.
Theresa May said G4S and Locog met at the Home Office on June 27 and said they were "experiencing scheduling problems" which could see a shortfall of "significantly less than 1,000" guards.
But the firm was "unable to specify the size of the shortfall" and only said they were no longer confident of reaching their workforce targets on July 11.
The details came in a letter from Mrs May to Labour MP Keith Vaz, chairman of the Commons Home Affairs Select Committee, and as another 1,200 troops were put on standby to provide Olympics security as the fallout from the G4S chaos continued.
But Culture Secretary Jeremy Hunt said the numbers of staff provided by the company were rising, and there was currently no need to deploy more military personnel.
Mrs May wrote: "On June 27 G4S and Locog attended an Olympics Security Board meeting at the Home Office and said they were experiencing scheduling problems. They warned of a possible temporary shortfall in G4S deployed number from July 1. G4S were unable to specify the size of the shortfall and could say only that it would be 'significantly less than 1,000'. G4S stated that the shortfall was mainly due to the failure to take account of the fact that large parts of their workforce would be unable to begin work before July 27."
Mrs May went on: "Locog and G4S were pressed to clarify the shortfall and factors which had created it urgently. The meeting considered a possible short-term and temporary call on the military contingency force (MCF) which had been created for Olympics purposes."
Mrs May admitted that permission to put "a small part of the MCF" 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, she added. Plans to put even more troops on standby were under way on July 6.
"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."
Mr Vaz said: "The Home Secretary told the House that she only became aware of a shortfall on July 11. However this letter clearly states they were warned of a possible shortfall in guards on the June 27 at the Olympic Security Board, two weeks before."