The International Olympic Committee's evaluation commission has begun arriving in London to inspect the city's bid to host the Games in 2012.
First of the 13-strong IOC team to arrive was South African delegate Sam Ramsamy, with the others arriving separately throughout the day.
Ramsamy was greeted at Heathrow airport by London 2012 chairman Lord Coe and Sports Minister Richard Caborn.
The four-day inspection begins on Wednesday and will be led by Coe.
London is competing against Paris, New York, Madrid and Moscow and the evaluation report will help IOC members decide the winner in a vote on 6 July.
Coe will lead the tour of London's plans and key sites, and the IOC team will also attend a special dinner with the Queen at Buckingham Palace.
An artist's impression of the statue to be erected in Trafalgar Square
More details on London's bid
Paris is considered the favourite, but Coe said: "People who watch this closely on a day-to-day basis think we have narrowed the gap quite dramatically in the last few months.
"But I think it is too close to call."
London's bid has been praised by the IOC's former head of marketing Michael Payne.
"London keeps raising its game. I am pleasantly surprised. There is a long way to go but this is the most open race for 20 years," Payne told the Evening Standard newspaper.
London has staged the Olympics twice, in 1908 and 1948. Attempts to bring the Games to Manchester in 1996 and 2000 both failed.
Many of London's key landmarks have been decked out with bid flags, and a statue of an athlete representing British pride and honour will be erected in Trafalgar Square on Saturday.
Coe's team hope their plans to regenerate a 500-acre area in the Lea Valley in the east of the city will go down well with the IOC.
They will take the inspectors to Stratford to see the main Olympic site, and use computer-generated technology to show what it would look like with all the key venues and transport hub in place.
"Technically, we have got the best bid simply because it is an opportune time," said Caborn.
"We've got the brownfield sites there and we've put the transport structure into the east end so we can move people from the centre of London to the Olympic parks in seven minutes."
The IOC inspection team, which has already visited Madrid, is led by Moroccan Nawal El Moutawakel, the first Muslim woman to win an Olympic gold medal.
It includes several technical experts, and Namibian sprinter Frankie Fredericks is the athletes' representative.
The members will be given presentations on 17 themes, including transport, security and London's Paralympic bid.
Coe's team are expected to show them planned venues like the new Wembley stadium, Wimbledon, Lord's and Horse Guards Parade, which would stage beach volleyball.
They are due to meet Prime Minister Tony Blair and the other main party leaders at Downing Street on Saturday before having dinner with the Queen and Prince Phillip.
The commission's verdict, which will be published in May, is not binding and there is no official ranking.
But it takes on added significance because new rules designed to cut down on corruption prevent other IOC members from visiting bidding cities.
A critical evaluation for any of the five rivals would be considered disastrous, although they will get to make final presentations just before the final vote in Singapore.
