A request from the Home Office to the Ministry of Defence had been expected, but the armed forces wanted ministers to make a more decisive move, mobilising the extra men and women immediately.
But an emergency meeting in Whitehall on Thursday afternoon involving officials from the MoD, the Department for Culture, Media and Sport, as well as executives from G4S, ended with a compromise.
Instead of being on seven days’ notice to deploy, 1,200 troops will be put on 48 hours’ notice. It is thought many of them will be from 20th Armoured Brigade, which is based in northern Germany.
Thursday’s announcement came a week after the Guardian revealed that an extra 3,500 members of the military were being rushed into Olympic duties, bringing the total number of armed forces personnel at the Games to 17,000 – almost twice the number in Afghanistan.
The MoD is anticipating that some, if not all, of the 1,200 will be needed at some point, and commanders would have preferred them to have been mobilised straight away and brought to London.
One Whitehall source said: “The military likes to work with certainties and plan ahead. Politicians prefer to leave things to the last moment.”
If G4S encounters more problems, officials made clear the Home Office may have to think again and make another formal request to the MoD for help.
“This should be the last call up,” said another source. “It would be hugely embarrassing if we had to go through this again.”
Announcing the decision, the culture secretary, Jeremy Hunt, said: “We must prepare for every contingency. We are therefore putting an additional 1,200 troops on standby, reducing their notice to move from seven days to 48 hours.
“They will remain in their current locations but can be called on if we need them during the coming weeks. We hope that will not be necessary but this is a sensible precaution. G4S numbers are rising and we are seeing an improvement in the company’s performance, which is to be welcomed.
“We are confident that we are on track to stage a great Games.”
An MoD spokesman added: “We are putting a further 1,200 troops on standby as a sensible and prudent measure to provide resilience to the Olympic security effort. The government has judged there is no current need to deploy these additional personnel who will only be called on to deploy if needed, but safety and security for the Games is the highest priority and so we must prepare for every contingency.
“No member of the of the armed forces will lose leave or be left out of pocket due to Olympics duties.”
The decision came as the row over the G4S debacle took a new turn.
In a letter to Keith Vaz, the chairman of the home affairs select committee, Theresa May admitted her department was warned of a possible temporary shortfall in G4S guards on 27 June.
More than 700 troops were deployed to cover the gaps at the time.
Vaz said he was surprised to learn this as it was a full two weeks before the home secretary told the Commons that the G4S problems had “crystallised” on 11 July.
“This letter clearly states that they were warned of a possible shortfall in guards on 27 June at the Olympic security board, two weeks before. We now know there must have been serious concerns as some 725 military personnel were deployed and contingencies started to be made before 11 July,” Vaz said.
May’s letter said that on 27 June, G4S and Locog, the Olympic organisers, attended a security board meeting at the Home Office and warned of “a possible shortfall in G4S deployed numbers” from 1 July.
May said that the company was unable to specify how many they were short only that it would be “significantly less [sic] than 1,000″.
The firm said they had failed to take account of the fact that large parts of their workforce would not be able to start work before 27 July.
The Home Office sought approval to put a small part of the military contingency force on 24 hours notice and by 9 July a total of 725 troops had already been mobilised.
The disclosure was made after the Labour leader, Earlier, Ed Miliband urged the government to rethink its plans to outsource more police services to private companies.
He said the Olympic raised wider questions about the outsourcing of policing services all over the country and he called for a halt until there had been a review of G4S’s ability to deliver.
The private security firm already has a £229m contract with Lincolnshire police which extends to running a police station. Police authorities in West Midlands, Surrey, Bedfordshire and Cambridgeshire all decided to pause their police privatisation contracts last week. But one police authority, Hertfordshire, has said it will not shelve a deal with G4S to take over its police support jobs.