London 2012: More troops deployed for Olympics
More than 1,000 troops who had been on standby to bolster security at the Olympic Games have been deployed.
Culture Secretary Jeremy Hunt said the 1,200 troops were to be used because ministers were clear that "we don't want to leave anything to chance".
Earlier this month 3,500 personnel were drafted in after security provider G4S admitted it was short of staff.
The new deployment decision was taken at a meeting of the cabinet's Olympics committee, chaired by David Cameron.
The move means 18,200 troops have now been deployed to the Games.
It is the latest step to strengthen security after G4S said it could not deliver enough guards.
Paul Deighton - chief executive of Olympic organisers Locog - said G4S had just under 6,000 personnel deployed for the Games to date.
Mr Hunt told an Olympics security briefing the next 48 hours were "critical" for security and transport".
Earlier, he said that the move to bring in extra troops was not because G4S's performance had deteriorated - he said it had actually improved.
"With three days to go until the opening ceremony, with an incredibly busy weekend, we don't want to leave anything to chance and we just decided that this is the right measure to take because for the public the most important thing is a safe and secure Games," he said.
"It's better to have those troops on the ground so that, were they to be needed, they can swing into action immediately."
In other Olympics news:
- Heathrow is having its busiest day for the Olympics so far as it handles about 217,000 passengers, including 1,200 athletes and coaches and more than 3,000 other Games-related arrivals
- Motorists face long queues on routes where modifications have been made to road layouts ahead of Wednesday's full introduction of 30 miles of Olympic traffic-only Games lanes in London. Up-to-date traffic information can be obtained from the BBC Travel news site for London and a website created to highlight likely disruption
- The Jamaican track and field athletics team, which includes the 2008 Olympics 100m and 200m champion Usain Bolt, are holding an open training session in Birmingham ahead of the Olympics
- Team GB is set to be officially welcomed to the Olympic Village later at a ceremony involving dancers, poets and rappers curated by the National Youth Theatre
- The British Olympic Association says it has asked triple jumper Phillips Idowu to supply details about his injury ahead of the Games after the 2008 Olympics silver medallist pulled out of the Great Britain athletics team's training camp
- The Olympic torch is travelling from Kingston, in south-west London, to Ealing, west London, as it continues its tour of the host city's 33 boroughs.
During a security briefing, Metropolitan Police Assistant Commissioner Chris Allison said the deployment of further troops offered extra flexibility.
"We don't know what's going to come round the corner, and that's the reality," said Mr Allison, who is the national Olympic security co-ordinator.
Mr Allison said the security operation had been designed to deal with four broad areas of risk to the Games: "The first is terrorism, the second is organised crime, the third is protest and the fourth is natural hazards."
He said while the existing terror threat to the UK was rated at "substantial", officers were policing to the level above that - "severe" - when an attack is highly likely.
"You can always downgrade your resources at the last minute - it's a lot harder to suddenly upgrade your resources."
Opening ceremony rehearsal
Mr Allison said the team dealing with Olympics-related fraud had so far made more than 200 arrests and he advised people to only buy tickets from authorised websites.
He also warned that while the right to protest was "an extremely important part of our democracy", it was a "conditional" right and therefore "does not give you the right to stop the Games happening".
On Monday, thousands attended a technical dress rehearsal of the opening ceremony.
They were urged to keep the details secret and not spoil the surprise for others by circulating images or videos.
People who attended Monday's rehearsal reported that the hashtag "#savethesurprise" was emblazoned on giant screens inside the Olympic Stadium.
It later trended globally on micro-blogging website Twitter.