One million British travellers planning to fly to Canada, the Caribbean and Mexico this year face the risk of being turned away at the airport – at the insistence of the US Department of Homeland Security.

New rules require British Airways and other airlines flying to certain airports outside America to submit passengers' personal data to US authorities. The information is checked against a "No Fly" list containing tens of thousands of names. Even if the flight plan steers well clear of US territory, travellers whom the Americans regard as suspicious will be denied boarding.

Simon Hughes, the deputy leader of the Liberal Democrats, told The Independent: "The concern by the US for its own security is entirely understandable, but it seems to me it's a whole different issue that American wishes should determine the rights and choices of people travelling between two countries neither of which is the US."

For several years, every US-bound passenger has had to provide Advance Passenger Information (API) before departure. Washington has extended the obligation to air routes that over-fly US airspace, such as Heathrow to Mexico City or Gatwick to Havana.

Now the US is demanding passengers' full names, dates of birth and gender from airlines, at least 72 hour before departure from the UK to Canada. The initial requirement is for flights to Toronto, Ottawa, Montreal and the Nova Scotia capital, Halifax – 150 miles from the nearest US territory. A similar stipulation is expected soon for the main airports in western Canada, Vancouver and Calgary.

Any passenger who refuses to comply will be denied boarding. Those who do supply details may find their trip could be abruptly cancelled by the Department of Homeland Security, which says it will "ake boarding pass determinations up until the time a flight leaves the gate ... If a passenger successfully obtains a boarding pass, his/her name is not on the No Fly list." In other words, travellers cannot find out whether they will be accepted on board until they reach the airport.

Canadian Affair, the leading charter operator between Britain and Canada, began supplying the data a week ago and 13,500 of its clients have complied with the demands. None has so far refused to provide the information and no one has been refused boarding.

Air Canada and British said they would comply with any new rules and The Independent understands that they will join the scheme in April. Flights to Mexico and Cuba - the Caribbean island closest to the US – are also included.

The US will have full details of all British visitors to Cuba, including business travellers, which could potentially be used to identify people suspected of breaking America's draconian sanctions against the Castro regime.

Neil Taylor, a tour operator who pioneered tourism to Cuba, said: "Imagine if the Chinese were to ask for such data on all passengers to Taiwan, and similarly if the Saudis were to ask about flights to Israel – would the US government understand?

"One also has to wonder how an American traveller in Europe would react if he were denied boarding on a flight from London to Rome because the German government had not received sufficient data from him."

Tony Wheeler, founder of Lonely Planet travel guides, said "This extension of the rule to include flights that never enter US airspace is scarcely credible. What on earth right does the US have to ask for passenger information if you're flying London-Havana?"

NOW BOARDING: WHO IS AFFECTED?

725,000: Number of British visitors to Canada each year. Airports affected: Ottawa, Toronto, Montreal and Halifax

300,000: Number of British visitors to Mexico each year. Airports affected: Mexico City and Cancun