John Ogilvie, a tunnel engineer who was commissioned by the City investment bank UBS to examine the project, says the long-term capacity is about half that claimed by Eurotunnel, owner and operator of the tunnel.
Mr Ogilvie suggests that Eurotunnel will be able to take a maximum of only 10 trains through the tunnel each hour. 'If it changes the mix of trains in the summer peak, it could possibly get 13 trains an hour through the tunnel,' he says. 'But 20 looks impossible.
'My findings have been on the stocks for some time now and Eurotunnel has yet to provide a detailed argument against them. For the sake of its investors and bankers it is time that it did so.'
Eurotunnel, which needs to raise a further pounds 1.3bn in the next couple of months, said that it would be detailing the basis for its revenue projections in the prospectus for its next share issue, due by the early summer.
Mr Ogilvie believes he has suffered professionally from taking such a negative view of one of the country's largest transport infrastructure projects. 'I have been unable to find somebody able to check and stand by my theories because there is so much moral cowardice around,' he says.
He arrived at his reservations over the capacity of the tunnel after taking into account four main points. He says that the trains' maximum speed is lower than originally planned; that freight trains running between passenger trains will slow the traffic; that steep gradients at the ends of the tunnels will reduce acceleration; and that the terminals were built too close to the tunnel ends to allow rapid acceleration.
Eurotunnel said that the company did not want to get into a debate with the engineer. A company spokeswoman said that she could not comment on somebody else's report, except to say: 'We are saying something different.'
Mr Ogilvie was commissioned by UBS in 1991 to write a report on Eurotunnel's capacity problems. His findings have influenced Richard Hannah, the firm's respected transport analyst, who has consistently taken a negative stance over Eurotunnel's shares.
The tunnel has cost around pounds 10bn to build, twice as much as originally predicted, and its opening has been delayed many times. This has been partly in response to concerns over safety and security.
Mr Ogilvie's report has never been published. Terry Smith, former head of company research at the investment firm, maintains that inquiries to test Mr Ogilvie's credibility were stopped in their tracks.
'He could be right or he could be wrong,' Mr Smith says. 'But that's not really the point. So far there's been no real debate about what he has to say.'