Britons hoping to visit the US will no longer be admitted under the Visa Waiver Program (VWP) if they have been to Iran , Iraq, Sudan or Syria in the last five years - nor will those who possess dual citizenship that includes one of those four countries.

US authorities said that the tightening of entry regulations, which come after November’s terrorist attacks in Paris , was to maintain the "highest standards of security".

But they will make transatlantic trips costlier and more time consuming for tens of thousands of UK citizens. A tourist visa currently costs $160 and requires applicants to both complete and online form and attend an interview at the US Embassy in London. There is currently a six-day wait for an appointment and a decision may take up to 60 days. The $160 fee is not refunded if applications are unsuccessful.

Dual-nationality British-Iranians, Syrians, Iraqis and Sudanese who currently hold US visa waivers from previous travel will have them revoked, and will have to apply for visas at the embassy.

In addition to the numerous UK passport holders with dual nationality who will be affected, many Britons who have simply visited one of the listed destinations on holiday will now be hindered.

While Iraq and Syria have been off-limits to most ordinary travellers for the last five years, much of Sudan remains safe to visit.

Furthermore, the Foreign Office lifted its advisory against trips to Iran last year. The country was named among Telegraph Travel’s 20 destinations to visit in 2014 and UK tour operators reported a surge in interest during the second half of last year.

Iran was named among Telegraph Travel's 20 destinations to visit in 2014 Photo: AP

There are a handful of possible exemptions. Those who travelled to Iran, Iraq, Sudan or Syria for humanitarian or journalistic reasons, for example, may still be able to apply under the VWP, but the Department of Homeland Security will decide on a case-by-case basis.

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The new rules officially came into effect yesterday, but a number of Britons – including BBC journalist Rana Rahimpour, who has dual British and Iranian nationality – have been prevented from boarding flights in advance of their introduction.

The majority of Britons will still be able to visit the US under the VWP. This entitles them to up to 90 days visa-free travel, but they must still apply for pre-approval using the online Electronic System for Travel Authorisation (ESTA). ESTA applications cost $14 and last for five years.