Britain may have to call up more soldiers to guard the Olympic Games after a failed private sector recruitment drive left an embarrassing hole in security.

The security fiasco and doubts over the ability of London's transport system to handle large numbers of visitors to the Games have overshadowed an event which the government still hopes will give recession-hit Britain something to celebrate.

The omens did not look good as athletes from across the world arrived in London this week, some tweeting frustration with transport delays amidst a media storm over security fears.

Passengers arriving for the Games could also face queues and chaotic delays after a union representing passport officials said its members had voted to back a strike.

And, after months of rain and floods, the Games have already earned the nickname of the Soggy Olympics in the British press.

"There are challenges and the military have stepped up to the plate and I salute them for it," Prime Minister David Cameron said during a visit to Afghanistan. "Let's not call it a soggy Olympics, let's call it a great Olympics."

The glitch came after security firm G4S said it could not provide a promised 10,400 security guards to watch over the Games venues, exposing the government to accusations of poor planning.

To fill the gap, the defence ministry called up an extra 3,500 troops - many just back from serving in Afghanistan - to take the armed forces contribution to 17,000 personnel.

More troops could now be deployed to control crowds at the Games if G4S struggles to find a minimum requirement of 7,000 staff. An extra 2,000 troops may be needed.

Defence Secretary Philip Hammond said more troops would be deployed if needed.

"If there is a requirement for more military support, it will be provided," he said.

Horses, cows, goats and chickens

Britain is keen to soothe security fears in a city where suicide bombers killed 52 people in attacks on the transport system the day after London was awarded the Games, in July 2005.

The latest victim of London's roads and rail network was the July 27 opening ceremony, due to be held in the evening, which has been trimmed to avoid a possible late-night stampede for trains and buses home, officials said.

A daredevil stunt bike performance was cancelled following rehearsals, with some reports suggesting the event was removed over fears that security checkpoints would be unable to cope with a rush of more than 60,000 spectators.

Due to be watched by a huge global television audience, the extravaganza will recreate scenes from the British countryside and includes a cast of 10,000 performers, 12 horses, three cows, two goats and 10 chickens.

"We need to make sure the show comes in on time, to make sure spectators can get home on public transport," a spokesman for London 2012 said.

Security

On Tuesday, G4S Chief Executive Nick Buckles suffered an excruciating grilling by irate lawmakers, agreeing during a parliamentary hearing that the failure to recruit enough guards had left the firm's reputation "in tatters".

"Let's be clear, if G4S don't fulfil their contract we will go after them for the money to make sure that they help pay for the military personnel that have been brought in," Cameron said.

Hugh Robertson, the minister responsible for the Olympics, said contingency plans were being drawn up for the extra troops, and the government would pay for the extra security to make up for G4S's failings.

London's underground trains and narrow, congested streets is under strain from commuters, tourists and shoppers at the best of times, but it must cope with an extra burden in the coming weeks.

In a piece of positive news for organisers, Britain's official statisticians said on Wednesday the Olympics had given a much-needed jobs boost to the economy.

However, the Olympics feel-good factor has yet to be felt by many in Britain, according to an opinion poll which showed excitement had fallen noticeably this week.

Some events have failed to attract the expected interest. Half a million tickets for Olympic soccer matches were withdrawn and stadium capacities reduced this week due to a lack of demand.

Transport problems have also contributed to a difficult last week before the Games. Taxi drivers brought traffic outside parliament to a standstill on Tuesday in protest at their exclusion from reserved Olympic traffic lanes.

Some relief for the organisers came after London bus workers accepted an offer of an Olympic bonus, ending the threat of a strike next week.