Britain's government has opted to deploy 1,200 more troops to protect Olympic venues, after a key security contractor failed to supply enough guards.
The fresh troops come only three days before Friday's opening ceremony and mean that some 18,200 UK military personnel are now involved in some capacity in securing the London Games - dwarfing the 9,500 British troops now in Afghanistan.
The decision followed a Cabinet meeting discussing venue security.
"On the eve of the largest peacetime event ever staged in this country, ministers are clear that we should leave nothing to chance," Olympics Secretary Jeremy Hunt said in a statement.
"The Government continues to have every confidence that we will deliver a safe and secure Games."
The government put the troops on standby a few days ago, but suggested that was merely a prudent contingency measure that was unlikely to be used.
The announcement on Tuesday is yet another embarrassment for security provider G4S, which has consistently failed to deliver on its Olympic contract.
Thousands of British soldiers have been sent on short notice to fill the gap in guards.
Some of the servicemen have seen their leaves cancelled while others have only recently returned from tours in Afghanistan.
The chief executive of G4S, Nick Buckles, has acknowledged that his company's failure to hire enough Olympic security guards had embarrassed the nation.
He made a grovelling apology last week when he was questioned by angry British MPs at Parliament, who have suggested that "sorry" wasn't enough.
Some MPs want the company, one of the world's largest security providers, to not only pay for all additional costs incurred by the government for bringing the extra troops in but also face financial penalties for the failing to deliver.
Britain's Prime Minister David Cameron has promised to "go after" G4S if they don't fulfil their contract to make sure the company helps pay for the cost of the additional military personnel.