As the Christmas travel rush gets under way, the US Department of Homeland Security (DHS) has changed the way it grants permission for holidaymakers and business travellers to travel to America.

Visa-free travel is permissible only with a valid ESTA – which stands for the Electronic System for Travel Authorisation.

While the US authorities have always advised that applications are made at least three days in advance, many travellers have had their ESTA granted almost instantly – including passengers who turn up at the airport without the permit.

But a news reminder on the front page of the official site warns: “Apply for ESTA no later than 72 hours before departing for the United States.

“Real-time approvals will no longer be available and arriving at the airport without a previously approved ESTA will likely result in being denied boarding.”

A valid ESTA does not guarantee admission to the US, with the DHS warning: “Upon arrival in the United States you will be inspected by a Customs and Border Protection officer at a port of entry who may determine that you are inadmissible.”

Two years ago the DHS added a question to the online form that asks about the applicant’s social media activities. The request, which is optional, is intended to identify “potential threats” or “potential deception or fraud,” but may also “validate legitimate travel”.

“If an applicant does not answer the question or simply does not hold a social media account, the ESTA application can still be submitted without a negative interpretation or inference,” says the DHS.

After Brexit, British travellers to Europe will need to apply for an ESTA-type permit, known as ETIAS, from 2021.

Travellers flying from or within the US over Christmas and the New Year are being warned to expect “stronger security measures”.

The DHS says: “Passengers may experience more checks and additional screening of their bags.