London faces an "uphill task" preparing its transport network for the 2012 Olympics, according to a report by MPs.
The report says disability access has not been properly considered
It said making journeys a pleasant experience and ensuring athletes, officials and spectators are not hit by delays will require a lot of work.
The report by the Commons Transport Committee said a 15% fall in traffic would avoid heavy congestion but it was unsure how this could be achieved.
Without good transport, the Games would fail, Labour MP Gwyneth Dunwoody said.
Ms Dunwoody, committee chairman, said: "The world's spotlight will be on London and if it fails to provide excellent transport for the Games, there will be no hiding place and no excuse."
Another concern highlighted in the report was a discrepancy in the capacity of "Javelin" trains from King's Cross to Stratford - Mayor Ken Livingstone predicts 25,000 passengers an hour while train companies say up to 14,000.
There are also fears over possible platform overcrowding at Stratford international station and the lack of consideration for access for disabled spectators, according to the report.