Its president Larry Pimentel said the ship, which is likely to be called Queen Mary II, would only sail under the Red Ensign if the company could reach suitable working agreements with UK shipping unions.

"If a UK flag doesn't make economic sense, we won't have it," said Mr Pimentel. "We would love to be able to use the British flag, but it depends on the unions. They would have to co-operate.

"Unions will have to look at things like vacation days, benefits, salaries, and working hours. It's more expensive to have British unions on the ship. I hope we can fly the British flag, but there are plenty of foreign-flagged ships all over the world today."

'Retain the flag'

Unions have reacted with fury to suggestions that the Red Ensign may be left off the vessel.

The leader of the RMT shipping union leader, Jimmy Knapp, said it was essential that the £300m ship should fly the British flag.

Knapp: "Happy to talk to Cunard"

"It is vital that we retain a British flag and a British crew for this prestigious project. The British public would expect no less," said Mr Knapp.

"The RMT will be happy to talk constructively with the company on how we can achieve this."

The masters' and officers' union, NUMAST, said flying a foreign flag on what would be the new symbol of the British merchant fleet would be disastrous.

NUMAST's spokesman Andrew Linington said: "This new ship is going to be the symbol of the British merchant fleet. To put a vessel like this under a foreign flag would be a disastrous move, really quite appalling."

The Queen Mary project could come into service in either 2001 or 2002, with the ship operating alongside the QE2.

Although the QE2 still sails under the Red Ensign, many Cunard and P&O vessels have been "flagged out" which means they sail under foreign flags or "flags of convenience", as they are known. This allows shipping companies to employ crew members who operate under different working practices, at cheaper rates.

Many liners sail under foreign flags

British unions have long bemoaned the flags of convenience tactic, claiming that safety standards were suffering from the pursuit of profit.

Cunard - now owned by the American cruise and holiday company Carnival - announced in June this year that it planned its "grandest ever" liner to recreate the golden age of sea travel.

The new ship will be the first new liner to be built for Cunard since the 67,000-tonne QE2 which was launched in 1967. The original 80,000-tonne Queen Mary was launched in 1935 and was in service until 1967 when she became a tourist attraction at Long Beach, California.

Conservative MP Sir Teddy Taylor has tabled a Commons questions urging Trade and Industry Secretary Peter Mandelson to persuade the unions to come to an agreement.

He said: "It would be tragic for Britain if this massive new Cunard superliner were not to sail under the British flag.

"Now is the time for a superlative effort to ensure that the Red Ensign does appear on this magnificent new vessel. Mr Mandelson must now intervene as a matter of the utmost urgency."