Transport plans for the 2012 Olympics rely too much on "highly speculative" assumptions 600,000 Londoners will flee the city for the games, MPs warn.
Public transport will play an important role in 2012
Expectations non-Olympic traffic will drop by twice the 8% it usually does in summer pose a significant risk, says the Commons transport committee report.
Up to 80% of passengers are expected to travel by rail, but contingency plans in case of problems were "embryonic".
But transport bosses say plans are on track and have hit every milestone.
Reporting on the draft transport plan, the committee said it hoped the next version of the plan would provide more answers.
MPs were concerned that contingency plans for things like power failure, security alerts and signalling problems were not well developed.
"It is crucial that the transport systems put into place are robust enough to allow for major failures in parts of the system, without the entire system collapsing," the report said.
It has been estimated general road traffic in London would need to drop 15% to avoid congestion during the Olympics - due to start on 27 July 2012.
But the committee was concerned to note that although 8% of people were expected to leave for holidays, as they would do in any year in late July, planners were following the "working assumption" that an additional 8% would leave to get away from the Games - apparently based on the experience of other Olympic cities.
The report said it posed "a significant risk to the transport plan".
Speaking later, committee chairwoman Gwyneth Dunwoody said the whole committee had reservations about the estimate.
"It completely distorts the calculation on how many people would be using the transport system," she said.
She said they thought it more likely that, "if something exciting is going on in the London area", people would stay and invite more people from outside the city to stay with them.
MPs also urged the Olympic Delivery Authority (ODA) to get on and start building the infrastructure, so that enough time was left for testing at the end.
They criticised the lack of detail about using the waterways to transport freight and people, as well as about park-and-ride schemes.
And there were also doubts about the extent to which Londoners, who are contributing a "significant part of the cost" of the Games, would benefit from a lasting transport legacy.
More effort should be made to provide a mechanised link between the two Stratford stations and to boost walking and cycling, they said.
Mrs Dunwoody said they did not intend the report to be entirely negative, but she had expected more answers on questions such as bus lanes and special lanes for athletes getting to the venues.
"Now is the time to be getting on with the difficult decisions, not just before the thing is scheduled to begin," she said.
During committee hearings, MPs heard that plans were generally 12 to 24 months ahead of what the International Olympic Committee expected.
Responding to the report, the ODA 's transport director Hugh Sumner said "Scrutiny and debate at this very early stage of consultation is important. However we remain confident in our preparations for 2012."
He said it was "unprecedented" to have a draft transport plan ready so early and that "every milestone" had been hit to date.
"Of course we are not complacent, of course there will be challenges ahead, but we have no doubt we will deliver a world class transport system for the Games," he said.
For the Conservatives, shadow sport and olympics minister Hugh Robertson said the government had failed to produce a "proper budget" for 2012, which was now having "a detrimental effect on every aspect of London's Olympics".
The final transport plan is expected to be published this summer, although it will be further revised in the light of experiences from the Beijing Games in 2008.