The Trump administration is considering making changes to the US visa-waiver system that could make it much harder for Europeans travelling to America.

Secretary of Homeland Security John Kelly said the existing rules, which do not require Britons to obtain a visa, should be reviewed amid heightened concerns about terrorism.

"We have to start looking very hard at that [visa waiver] programme,” he said.

Mr Kelly added that he was not considering scrapping the system entirely, but said the current set-up needed to be looked at again.

"Not eliminating it and not doing anything excessive,” CNN Money reported him saying during a speech in Washington, “but look very hard at that programme”.

The comments come after airlines warned of a ’Trump slump’ that saw international travel to the US drop 6.5 per cent in the eight days following the President’s proposed travel ban in January.

Online searches for flights to America also dropped by 17 per cent in the same week.

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It resulted in an estimated loss of $185 million (£145 million) in business travel bookings from 28 January to 4 February, as calculated by the Global Business Travel Association.

Mr Kelly said he feared Isis fighters were using the visa-waiver system to sneak into the US, warning America was the “Super Bowl in terms of terrorists…that's where they wanna come [sic]”.

The visa-waiver programme enables most citizens of participating countries to travel to the US for up to 90 days without first obtaining a visa.

Travellers must first have authorisation under the Electronic System for Travel Authorization (Esta) system, a faster and cheaper process that takes 72 hours and can be completed online.

In 2015, the rules were tightened so that nationals of visa-waiver countries who had been to Iran, Iraq, Libya, Somalia, Sudan, Syria, or Yemen on or after March 1, 2011, were no longer eligible.

Around 14 million European citizens visited the US without a visa in 2015.

In March, the European Parliament voted to end visa-free travel for Americans within the EU.

European Parliament votes to end visa-free travel for Americans

It came after the US failed to agree visa-free travel for citizens of five EU countries – Bulgaria, Croatia, Cyprus, Poland and Romania – as part of a reciprocity agreement.

US citizens can normally travel to all countries in the bloc without a visa.