London would struggle to stage the Olympics without the much-delayed £4bn Crossrail project, according to a leaked Treasury document.
Crossrail would link Heathrow to Stratford
It also warns that security costs to hold the 2012 event could add £100m to the multi-billion cost.
The paper, commissioned by the government and written by engineering consultants Arup, said that London's transport network would struggle with the volume of spectators who could attend the Games unless events were staged outside peak travel times.
Crossrail, a rail scheme which would link Heathrow airport in the west with Stratford in the east was first mooted in the early 1990s.
Although the Strategic Rail Authority (SRA) and Transport for London (TfL) are preparing the business case for the project, no commitment has been made yet to finance it, nor has any decision been taken on the exact route.
Channel 4 News reported that the Treasury consultation document, from last year and entitled "London Olympics 2012 - Costs and Benefits", said: "Without Crossrail and without
event rescheduling ... the network cannot cope with the expected levels of demand."
The report suggested that the security operation in the post 11 September environment could top £130m.
The Treasury said it did not comment on leaked documents.
But at the weekend Culture Secretary Tessa Jowell said that no Olympic planning had been predicated on Crossrail.
Shadow transport secretary Tim Collins said: "This report confirms what the Conservatives have warned - London's Olympics bid's prospects will be significantly damaged the longer Transport Secretary Alistair Darling dithers over whether to give the green light to Crossrail."