Some 3,500 extra troops are to be drafted in for security duty at the Olympics amid fears that private firm G4S will not be able to supply enough personnel, Defence Secretary Philip Hammond is to say today.
The move, a major embarrassment for organisers Locog, comes as Mr Hammond appears before the Commons Defence Committee to explain how the Army is to lose 17 major units - including five infantry battalions - in the biggest re-structuring of the service for decades.
Some 17,000 troops will now be involved in the Olympics, with 11,000 of these involved in the security of more than 30 sporting venues and some 70 non-competition venues, including car parks and hotels.
Military personnel will also be involved in specialist support roles including air security, search teams, communications and logistics, among others.
Mr Hammond will give further details to MPs on Thursday.
It comes as security-providers G4S admitted it was experiencing "some issues in relation to workforce supply and scheduling" and had accepted that the government was turning to the military for extra help.
"Staff taken on don't yet know the first thing about any of the procedures, and that poses a huge security risk. How can a risk management company not have any continuity plans in place for the Olympics?", a manager told Channel 4 News.
"This has been an accident waiting to happen. The Home Office has waited to make a decision on this because G4S has been saying it is all in hand. But we've run out of time."
Additionally many of the soldiers due to join the London 2012 were expecting to go on summer leave after returning from Afghanistan
Retired Colonel Richard Kemp, a former UK commander in Afghanistan, said the development would hit troops "very hard indeed".
"Many of the soldiers that are coming - this extra 3,500 - I understand are soldiers who have just returned from Afghanistan or about to deploy to Afghanistan, so they are people who I imagine are getting ready to go on leave with their families, a well-deserved leave perhaps after six months away on operations or training for future operations, and this will hit them very hard indeed," he told BBC Radio 4's Today programme.
"As always when you give any part of the armed forces a task they will do it extremely well, extremely professionally and with a smile on their face, I have no doubt about that, and they recognise the importance of what they are being asked to do - they won't skimp at it, they won't regard it as a trivial task, I don't think.
"But we shouldn't forget also that many of these soldiers are people who have been told in the last few days that they are going to be made redundant, that their regiments are being scrapped and they are under great pressure already. The wider morale in the armed forces now is very fragile and this will simply add to that fragility."
Meanwhile border staff with only basic training and little immigration experience are being used to cut queues at Heathrow Airport during the Olympics, a watchdog warned on Thursday.
Some staff "remained concerned about the potential risks of employing staff on the immigration control who had received only basic training and who had no immigration background/experience", John Vine, the independent chief inspector of borders and immigration, said in his report.
A G4S spokeswoman said: "Our planning with Locog and other security agencies allows for a variety of contingencies which have been reviewed in the build-up to the Games.
"We accept that the government has decided to overlay additional resources.
"We remain committed to keep London 2012 safe and secure."
Home Secretary Theresa May was forced to defend G4S in the Commons on Monday, saying she was "confident our partners will deliver a safe and secure Games".
More than 100 different venues need to be protected during the Games, she told MPs.
Tessa Jowell, Labour’s Shadow Olympics Minister said the deployment of troops was clearly a “serious problem.”
“We need to know why the problem has emerged so late in the day and precisely what has been agreed to. 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. “
Some 13,500 troops are already involved in a wide variety of Olympics-related roles across the UK, including 7,500 for venue security.
Today's move will boost the number of servicemen and women involved in venue security to 11,000 and the overall number of troops involved in the Olympics to some 17,000, it is understood.
Along with venue security, military personnel will also be involved in specialist support roles including air security, search teams, communications and logistics, among others.
A spokeswoman went on: "We have agreed to offer help to G4S by revising the level of military support.
"The government are committing £553 million for venue security and we remain confident that we will deliver within budget."
She added: "The focus of the Government and everyone involved is on delivering a safe and secure Games.
"G4S are Locog's lead contractor for venue security and are being supported by the military.
"Our approach is intelligence-led and risk-based, and we retain the ability to be flexible in our response."
Former Commander of British Forces in Afghanistan Clonel Richard Kemp tweeted:
A spokesman for London Mayor Boris Johnson said: "The Mayor takes the issue of Olympics security extremely seriously.
"Having the finest, bravest servicemen and woman in the world at our disposal during the Games should be a source of great comfort."
Labour said on Thursday it was seeking an urgent Commons statement on the situation by the Government.
Mr Vaz said he had asked G4S chief executive Nick Buckles and its chairman Alf Duch-Pedersen to appear before the committee to explain why they do not have sufficient staff to provide security at the Olympics.
"I have written to G4S to inform them that the committee expects to see them next week," Mr Vaz said.
"Considering the assurances we have been given in the past this is very serious and we expect a full explanation from a company that not only have the Olympic contract, but receive hundreds of millions of pounds from the Home Office and other Government departments each year."
BBC Radio 4 Today
Liz Kendall MP