Details of an additional 3,500 troops expected to be provided to help with 2012 London Olympics security are set to be announced by ministers later.
It comes after it emerged that UK armed forces are on standby to provide more troops in addition to 13,500 already agreed amid fears private contractor G4S may not have enough trained staff.
G4S said it had "some issues in relation to workforce supply".
Labour's Tessa Jowell said there was "clearly a serious problem".
The shadow Olympics minister said there was a need to know why the problem has emerged so late in the day and precisely what has been agreed to.
Meanwhile, the chief inspector of borders has raised fresh concerns about the border controls at Heathrow in the run up to the Olympics.
The Home Office said Defence Secretary Philip Hammond would announce more details about the extra troops on Thursday.
It could result in summer leave for some troops being cancelled.
clear that the armed forces are being asked to step in.
The move could come as an embarrassment to G4S.
Whitehall sources are keen to stress that there will be no negative impact from all this on security at the Games, pointing out that the plan was always to have a mix of civilian and military personnel guarding the Games, with the contingency plans being activated as an "insurance policy".