In future, a one-way ticket on the 12-mile link from Heathrow airport to Paddington station in west London may cost as much as £12.

The fare compares with a single fare of £9.50 on the 26-mile Gatwick Express which runs from London Victoria station to the West Sussex airport.

That works out at 36p a mile.

A single London Underground ticket from Heathrow to Paddington costs £3.30.

BAA launched FastTrain -- a forerunner of the link -- on Monday.

FastTrain will not be running right into Heathrow from Paddington because work at the airport end has not yet been completed. Instead passengers will have to take coaches from a point two miles from the airport.

FastTrain is free to passengers with airline tickets from noon on Monday until next Sunday. After that, the fare will go up to £5 each way.

Not competing with the Underground

A spokeswoman for airport operator BAA, which has built the Heathrow link said on Sunday: "The single fare will be about £10-12 and a final decision will be made soon."

She denied the price was prohibitively high and said: "We are not competing with the Underground but supplying a fast service to Heathrow. We have also invested a large sum in this project."

The Heathrow Express, which is due to open in June will take 15 minutes compared with about an hour on the Underground.

Six million passengers in first year

Heathrow Express expects to carry six million passengers in its first year -- a figure which should result in 3,000 fewer airport-related cars and buses per day on the roads to the west London airport.

But there has been criticism that Paddington is not the ideal terminus for the Express, being too far out from central London.

There are plans to link the Express with St Pancras station in London in 2000 or 2001.