London must make a start on building work for the 2012 Olympic Games, a former president of the International Olympic Committee has urged.
London won the bid in July 2005
Juan Antonio Samaranch told the BBC 16 months had passed since London won the bid and time was "going very fast".
But developers said land was being prepared and buildings demolished, with building work due to begin in mid-2008.
Mr Samaranch's comments come after organisers said the cost of the Games could rise by £1.5bn to £5bn.
Olympic rowing gold medallist Sir Steve Redgrave said he could not understand how estimates could be so far out.
Culture Minister Tessa Jowell and Lord Coe, chairman of the London Organising Committee, are visiting Barcelona to see how the Spanish city benefited from hosting the Games in 1992.
Mr Samaranch was head of the Olympic movement for 21 years and was succeeded by its current head, Jacques Rogge, in 2001.
Although calling for building to get under way, he did say he was not worried about work being completed on time.
"In six years you can build many, many things," he said.
"I think people from London said they need two years for the preparations and four years for the works, and we are in that first two years."
The Olympic Delivery Authority (ODA) said builders will start constructing an aquatic centre, the main stadium and fitting out the athletes' village in 2008.
"It's hard for people if they can't see physical buildings but we are bang on target," an ODA spokeswoman said.
On Wednesday, the Mayor of London Ken Livingstone said the total bill for the Games could exceed £5bn.
The rise is being blamed on extra security, an unexpected VAT bill and government demands for a contingency fund.
Speaking on BBC Breakfast, Sir Steve Redgrave said: "One of the things that was always talked about at the time was that we were not going to go over budget and that the time and effort would be put in to make sure it did not go over."
"I find it a little disappointing. How can they not bring VAT into the whole programme?"
Liberal Democrat leader Sir Menzies Campbell said the government had to shoulder the extra costs.
"We have agreed to take on this tremendous responsibility - we have to pay for it.
"The government having been so closely involved, I don't think it can now step back and say this is the responsibility of the organising committee."
Organisers and the government are currently reviewing the original 2004 budget, expected to be set next February.