With the final, high-speed stretch into London's redeveloped St Pancras station due for completion next year, a report by the all-party Public Accounts Committee will criticise the forecasts on which the project was based. Hit by the terror attacks of 9/11 and 7/7, and by the growth of low-cost air travel, annual passenger revenues are expected to be nearly £40m lower than first predicted. Construction costs for the rail link have also been well above the levels initially budgeted, according to the committee.

Senior Department of Transport officials sought to reassure committee members at a hearing late last year that, since construction of the rail link was nearly finished, the government was unlikely to have to provide a major further cash injection for the 15-year, £5bn project. But the committee is concerned that, in addition to a government commitment to provide up to £285m to meet last-minute cost overruns, the future finances of the project will hinge on getting hundreds of thousands more people to use the new high-speed railway.

The link will cut rail travel time from London to Paris to 2 hours 15 minutes and bring a Brussels trip down to less than two hours. But the report is expected to warn that, unless this leads to a major increase in passenger numbers, taxpayers could end up having to provide millions of pounds of additional support for the project.

At last year's hearing, committee members were deeply sceptical about the prospects for an early rebound in passenger numbers. The committee's chairman, Tory MP Edward Leigh, suggested it was more likely that passenger forecasts would again have to be revised downwards.

The first mainline railway to be built in Britain for more than a century, the new link was one of London's selling points in its successful bid for the 2012 Olympics. In the wake of a separate Commons report last year which described the economic case for the project as 'marginal', the government has emphasised its broader benefits in terms of jobs, housing and overall 'regeneration', both around King's Cross-St Pancras and in Stratford - with its Eurostar terminal in the heart of the east London area being redeveloped for the Olympics.

Transport Department officials have also stressed the benefit of finally giving Britain a high-speed railway of the kind that other European countries already have.

With the link expected to meet its 2007 completion target, officials have argued that it has been far better managed than other major projects such as London's Jubilee Line extension, which came in nearly two years behind schedule and ran £1.4bn over budget.