British authorities insisted Thursday that Olympic security would not be compromised after having to draft in an extra 3,500 troops because a private security firm failed to provide enough guards.
The embarrassing development adds to worries about Britain's preparations for the London Games, which officially open on July 27, as fresh problems also emerged with transport and border security for the event.
Home Secretary Theresa May made an urgent statement to lawmakers confirming that private contractor G4S could not supply all the security staff it had promised due to recruitment and training problems.
"Let me reiterate that there is no question of Olympic security being compromised," the interior minister told lawmakers.
She said the government had built in contingency plans to what is Britain's largest security operation since World War II but that "concerns have arisen about the ability of G4S to deliver the required number of guards".
"We have now agreed it would be prudent to deploy additional military support to provide greater reassurance," she said.
May said she was confident the government would be able to stick to its security budget for the Games, which stands at £553 million ($877 million, 662 million euros).
A total of 17,000 troops will now be involved in the Olympics.
The extra deployment of 3,500 is equivalent in size to more than a third of the current total British military force of 9,500 in Afghanistan, and comes as the armed forces are facing widespread cuts.
But Defence Secretary Philip Hammond said it would have "no adverse impact on other operations."
He added in a statement that all soldiers involved would receive their full leave entitlement and compensation if they had had to change any arrangements.
May said the government was now donating 10,000 Olympic and Paralympic tickets to the armed forces, as well as 7,000 tickets for dress rehearsals of the Olympics opening and closing ceremonies.
But opposition lawmakers demanded an explanation for the change which comes just over two weeks before the start of events despite Britain having had seven years to prepare.
"G4S has let the country down and we have literally had to send in the troops," Keith Vaz, the Labour chairman of the home affairs select committee, told parliament.
Labour home affairs spokeswoman Yvette Cooper said the announcement "really looks like another huge Home Office shambles".
It was unclear what would happen to the reported £300 million fee the government had paid G4S for Olympic security.
Separately a parliamentary committee report published on Thursday found that Britain's spy agencies have been placed under "unprecedented pressure" by preparations for the Olympics, leaving the country at risk from other threats.
The Intelligence and Security Committee oversees the work of MI5, Britain's domestic intelligence agency, the foreign spy agency MI6, and the electronic eavesdropping agency GCHQ.
It praised them for working against potential threats to the Games from Al-Qaeda, Irish republican dissidents and "hostile states".
But the report said committee members remain "concerned at the risk that is being taken in some areas and the vulnerability of the UK at this critical period."
In a far cry from the image of fictional British spy James Bond, it added that one particular difficulty for the intelligence agencies was arranging childcare over the British summer.
Meanwhile an independent watchdog warned that border staff drafted in to cope with the influx of visitors to Britain for the Games may lack proper training and immigration experience.
In another pre-Olympics scare, the main M4 motorway between Heathrow Airport and London has been closed for repairs to a cracked viaduct. Britain's Highways Agency said it expected the road to reopen in time for the Games.