“The last thing you want to do is to apply at the last minute, sit in airport purgatory while you might get it in time, or worse, be denied boarding your flight because you didn’t receive approval,” said Katy Nastro, spokesperson for the travel app Going, which helps users get deals on flights.

The potential for travel disruption could be big. The United States-UK market is one of the largest international air routes in the world, with more than 20 million people flying between the countries last year, according to data from the US International Trade Administration.

Airlines offered around 150 daily flights between the countries this past northern summer, according to data from aviation analytics firm Cirium Diio.

The new UK requirement is part of a larger global shift towards digital border controls. Australia and the US already require foreigners who do not need a visa to register online for a small fee. And the European Union plans to implement its own digital entry requirement, or European Travel Information and Authorisation System (ETIAS), soon.

What makes the UK’s ETA different is the requirement that even connecting passengers secure one.

Virgin Atlantic Airways, one of the largest airlines connecting the US and the UK said in an emailed statement the transit requirements risked “putting the UK and its respective carriers at a competitive disadvantage”. It added that the EU’s ETIAS plan exempts transfer passengers from the digital certificate.

Someone flying from, for example, Washington DC to Mumbai, India, has multiple connecting flight options, including Virgin via London. With the new ETA rules, travellers now have an extra digital hoop to jump through for the trip if flying via London rather than Dubai or Frankfurt.

A spokesperson for London’s Heathrow Airport, the busiest in the UK, said in August that the airport had lost an estimated 90,000 transfer passengers as a result of a pilot ETA programme covering six Middle Eastern countries.

A UK Home Office spokesperson told the Guardian in August that the “Government is continuing to keep the requirement for transit passengers to obtain an ETA under review”.