London's creaking transport system was the sole blot on an otherwise squeaky-clean 2012 evaluation report by an IOC inspection team, it was announced at the end of their three-day visit yesterday.

“You cannot change the city, or enlarge the streets, and there will be people coming to London and going from one place to another,” Denis Oswald, the chairman of the IOC's Co-ordination Commission, said as construction work started ahead of schedule for the 2012 Games. “But transport is always a challenge in an Olympic city and London has the best experts on board who will find solutions. Fortunately, the Games are in August, when a number of people will be on holiday, so there will be fewer cars.”

It is hoped that the capital's gridlocked roads will be eased by dedicated Olympic lanes for athletes and officials travelling between events.

Oswald marked the city 9.75 out of 10 and praised the “unprecedented” preparation for the world's largest sporting event. The inspection covered all aspects of the Games, from sporting venues through to security.

“Nobody's perfect,” he said, referring to the missing 0.25. “The level of detail in the presentations four years before is unprecedented and we have full confidence that we will have an excellent Games. We have seen the shape of the stadium, we have seen something coming out of the ground and that is a good indication that the Games are taking shape. From what we have seen we are very confident we will have excellent facilities.”

Lord Coe, the chairman of the London Organising Committee of the Olympic Games (Locog), said that the start of construction yesterday - three months ahead of schedule - on the £496million Olympic stadium was timed for maximum impact.

Gordon Brown met the 18-strong IOC inspection team at the site where 4,000 concrete columns will be inserted into the soil as foundations for the 53-metre high stadium. By the end of the year, floor slabs will be laid for the base of the stadium bowl and the lower-tier structures and the columns that support the pedestrian concourse level will be built. Early next year the erection of the steel structures that will support the roof will begin. The stadium, which will seat 80,000 during the Games and 25,000 afterwards, should be finished in 2011.

Oswald defended its simplicity - described by critics as lacking in creative inspiration compared with Beijing's spectacular Bird's Nest stadium - as “perhaps an example for the future” because it took realistic account of its post-Games use.

Boris Johnson, the Mayor of London, who was critical of the public spending on the Olympics during his election campaign, made his first visit to the 500-hectare Olympic Park. “I'm blown away by this,” he said. “Anyone who has had anything to do with British builders will know what an astonishing achievement we are witnessing.”

Oswald reserved a word of warning for the London organisers that the spotlight would shift to them after the Beijing Games in August. “The Mayor will receive the Olympics flag and the focus will be on London,” he said. “The pressure will move from Beijing to London.”

Responding to domestic concerns about London's £9.3billion Olympic cost to British taxpayers, he said that the decision to build expensive infrastructure - such as the £303million swimming and diving facility - was for the Government, not the IOC. “If it can be cheap, fine, but if they feel there is a need for a fantastic aquatics centre, it is their decision,” he said. “Atlanta [1996] didn't spend a cent. Athens [2004] spent a lot of money.”

The £2billion operational budget for the 16-day event will be covered by an IOC contribution from revenues generated by its central marketing, sponsorship and television deals, plus a fund raised by Locog from the private sector.

The £9.3billion budget refers to the cost of building the sports venues, regenerating a toxic wasteland in the lower Lea Valley, VAT, security and a contingency fund to cover unforeseen events. This is covered by the public purse. Any increase in the budget will not affect the running of the Games, in the IOC's view, because the Government is the guarantor of last resort and there is a fixed deadline for the completion of infrastructure.