South west MPs have joined forces to fight for the sleeper

It costs about �5m a year to run the service from Paddington to Penzance.

However income from ticket sales is "certainly less than 50% of costs", the Department for Transport said.

Cornish MPs are leading a campaign to save the sleeper amid fears it could be withdrawn next April by whoever wins the new Greater Western franchise.

A spokesman for the Department for Transport said: "We don't accurately know the cost of running the sleeper service.

This is a watershed decision for the Secretary of State. Andrew George, St Ives MP

"We have asked the bidders for the Greater Western franchise to give us separate costings for the sleeper. We suspect it could be done for better value for money, but no decisions have been made."

Bids for the Greater Western franchise, which combines the services currently offered by First Great Western, First Great Western Link and Wessex Trains, must be submitted by September.

The Department for Transport will choose the successful bidder and the new franchise will start on 1 April.

Andrew George, the Lib-Dem MP for St Ives, said: "This is a watershed decision for the Secretary of State.

"We accept that across a whole service, some elements will be more economically viable than others.

"But if the Secretary of State believes that he can strip out the less viable component parts without looking at the service as a whole then the very mainline service through Cornwall is under threat along with the branch lines."

The sleeper train is popular with business travellers from Cornwall because it means they can arrive in London in time for early meetings.