Democratic and Republican members of Congress will tomorrow join forces in support of a bill to end the decades-old ban on Americans travelling to Cuba, a move that could mark the beginning of a thaw in Washington-Havana relations.

The changing attitude towards Cuba contrasts with decades of outright hostility towards Castro, who claims he has been the target of CIA assassination attempts.

Two of the sponsors of the bill, the Democrat member of the House from Massachusetts, Bill Delahunt, and Jeff Flake, a Republican House member from Arizona, will introduce it at a press conference on Capitol Hill tomorrow. The bill is co-sponsored by 118 other House members.

Sarah Stephens, director of the Centre for Democracy in the America, which has been campaigning for normalisation of relations, asked the prospect of Americans on Cuban beaches before the end of the year, said today: "It is within the realms of possibiliity."

The US has had sanctions, both on travel and trade, in place against Cuba since 1962, three years after Castro took power. The US argument was that denying Castro revenue from trade and dollars from tourists would undermine the Communist government.

President George Bush tightened the sanctions, with new restrictions on family travel for Cuban-Americans. Obama has ordered a review of Cuba policy but in the meantime has de facto reversed the Bush changes by removing funding for enforcing the ban on Cuban-American family travel.

New relations with Cuba are expected to surface later this month when Obama attends the Summit of the Americas in Trinidad.

Americans face a $7,000 fine if they travel to Cuba. Exemptions are made for some journalists and academics who request special permission from the state department.

But some Americans flaunt authorities and go to Cuba via Canada, Mexico and the Bahamas. There are no direct flights from the US. About 50,000 Americans travelled legally to Cuba last year and an estimated 25,000 travelled illegally.

Visitors from Canada, including Americans ignoring the law, make up the biggest number of tourists to Cuba, with the British in second place. A US study predicted about 3 million Americans would visit Cuba each year if the ban was to be lifted.

One of the arguments put forward by members of Congress in favour of lifting the ban is that Americans are free to visit other countries regarded as communist such as China and Vietnam.

Delahunt, in a statement, said: "During his election campaign, now-President Barack Obama pledged to allow unrestricted travel by Cuban-Americans. He can do this by executive order, with no action needed in Congress. But for average Americans to exercise their right to travel to Cuba, Congress needs to change the law."

A bill similar to the House one was announced at a Senate press conference yesterday, and is co-sponsored by 20 members from both the Democratic and Republican parties.

Byron Dorgan, a Democratic senator, said: "I think we've finally reached a new watermark on this issue. At some point this is a policy that is no longer justifiable." Dorgan claims there is enough support to get the bill through.

Steve Clemons, a director at the New America Foundation, a Washington-based think-tank that favours ending the ban, today predicted that passage through Congress was more likely next year than this.

Mel Martinez, a Republican senator from Florida, where the hard core of anti-Castro Cuban immigrants are based, vowed to oppose it: "This is the time to support pro-democracy activists in Cuba, not provide the Castro regime with a resource windfall." In spite of Martinez's hostility, the younger generation of Cubans in Miami tend to be less hostile than their parents and are keen to visit.

The prospect for an improvement in relations with Cuba has been helped in policy-making circles in Washington by Castro standing down in favour of his brother Raul, who has introduced some reforms.

Vice-president Joe Biden, on a visit to Chile at the weekend, said there were no plans to lift the trade ban. But campaigners in favour of normalistation predict a gradual process in which the lifting one ban will open the way for a gradual lifting of all.