LONDON, July 18: Britain was forced to make more last-minute changes to the London Olympics on Wednesday, putting 2,000 extra troops on standby and shortening the opening ceremony to avoid transport problems.

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

“There are challenges and the military have stepped up to the plate and I salute them for it,” Prime Minister David Cameron told reporters during a visit to Afghanistan.

The glitch came after security firm G4S said it could not deliver a promised 10,400 security guards to watch over the 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 where Cameron was on a visit on Wednesday — to take the armed forces contribution to 17,000 personnel.

“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.

The latest victim of London's roads and rail network was the Olympic opening ceremony which has been trimmed to avoid a possible late-night stampede for trains and buses home.

Director Danny Boyle, the Oscar-winning filmmaker of “Slumdog Millionaire,” has been forced to cut parts of the Olympic opening ceremony — including removal of a stunt bike sequence — to make sure the show finishes on time and spectators can get home by public transportation.

London organisers said Boyle was “tightening” the ceremony by up to 30 minutes to ensure the show, scheduled for three hours, concludes between midnight and 12:30 a.m.

“This is like any other piece of film you would make, things end up on the cutting room floor,” London organizing committee spokeswoman Jackie Brock-Doyle said.

Due to be watched by a global television audience of a billion people, the July 27 extravaganza will attempt to recreate scenes from Britain's idyllic countryside and includes a cast of 10,000 performers, 12 horses, three cows, two goats and 10 chickens.

London's network of underground routes and narrow, cluttered roads is under strain from commuters, tourists and shoppers at the best of times, but now it must cope with an unprecedented burden in the coming weeks.

With the eyes of the world on the Games, a British government official said the London Olympics were the country's best chance to sell itself for more than 150 years.

Alan Collins, the managing director for Olympic Legacy at UK Trade and Investment, said he was confident they would help Britain attract 6 billion pounds (7.6 billion euros, $9.4 billion) in investment by 2016.

“This is the best opportunity since the Great Exhibition of 1851 to show off Britain,” Collins told a press conference.

Meanwhile, Britain's official statisticians said on Wednesday the Olympics had given a much-needed jobs boost to the economy. They did not specify if that increase came from the security sector.

However, the Olympics feel-good factor has yet to be felt by many in Britain, according to an Adecco poll which showed excitement had fallen one point to three out of 10 this week.

But with nine days to go until the Games begin there was at least one bright spot as forecasters said the sun was set to shine on the Games after Britain's wettest summer in 100 years.

Britain's tourism chief meanwhile hit out at the country's “utter obsession” with the weather, brushing off concerns that weeks of heavy rain would deter visitors to the Games.

“Frankly, people do not come here to lie on a beach,” said Sandie Dawe, chief executive of Visit Britain. “Anyway, the sun is going to come out — have you seen the long-range weather forecast?”

The Met Office, Britain's national weather service, said some much anticipated sunshine would return on Sunday and that southern England would enjoy dry weather next week, in time for the opening ceremony.—Agencies