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British tourists flying to the US risk being turned away from flights to the country unless they hold the latest biometric passports.

The warning came as the US Department of Homeland Security which confirmed that all travellers to the country must have an e-passport to use the visa waiver system from April 1.

This means that anyone flying to the US must hold a passport that contains an electronic chip – even if they have been granted an electronic visa.

Reports of UK tourists being caught out have started to emerge, The Guardian reported.

The change to US visa requirements was made late last year under the US Terrorist Travel Prevention Act 2015, following the San Bernardino shootings and came into effect at the start of last month.

But many tourists appear to be unaware of the change and have found their chance of flying to the US thwarted at the airport, when airlines reveal that their passport will not be accepted.

More than a million non-biometric UK passports are estimated to still be in use, and there are fears that the change has had insufficient publicity.

On the Foreign and Commonwealth Office travel advice website there is only one small and largely obscure reference to the new requirements for the US, and no effort to highlight the change in a way that would alert the average traveller, according to the newspaper.

Travel agencies have been warning their clients flying to the US, but those travelling independently have been caught out since little effort is made by airlines to warn passengers of the new visa requirements.

Airlines and operators say that passengers are responsible for ensuring they have all the documentation required for their trip.

As part of its restrictions the US also implemented changes to prevent foreign nationals who have travelled to Iran, Iraq, Libya, Sudan or Syria from entering the country without a visa. It also denied automatic visa waivers for foreigners who have dual Iranian, Iraqi, Sudanese and Syrian citizenship.

Abta warned members of potential problems in January.

A spokesman said: “Any customers who may have been caught out by this would have been more likely to be travelling independently.”

A spokesman for US Customs and Border Protection, said it had sent emails, where possible, to Esta holders, if their passport details were no longer valid.

Travellers should allow at least three weeks to obtain a new passport at the standard prices, although any remaining time on their existing document will be added to the new one.

Anyone needing a passport more quickly needs to make an appointment at a passport office in London, Liverpool, Belfast, Durham, Glasgow, Newport or Peterboroug and be prepared to pay at least £128.