Traveling to the U.K. next year? You need to get authorized online.

Americans traveling to London to see Big Ben, or maybe just changing planes en route to Cairo, will soon need to register online with the British government before they start their journey.

Beginning Jan. 8, all visa-exempt travelers landing at an airport in the U.K. with non-European passports will need an “electronic travel authorization,” or ETA, before they depart. The application, which opens for Americans on Wednesday, costs 10 pounds, or about $12.50. It can be done online or via an app. Once the ETA is approved, it is valid for two years and good for multiple entries of up to six months in Britain.

The ETA program expands to all passports, including European ones, on April 2. Citizens of the U.K. and Ireland, as well as those with valid U.K. visas, are exempt.

The new U.K. travel requirements create more work for Americans used to unhampered transatlantic travel. The new ETAs could take up to 72 hours to process, throwing a potential wrench in any 11th-hour plans for fliers who were long able to hop on a plane at a moment’s notice.

“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 U.S.-U.K. 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 U.S. International Trade Administration.

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

The new U.K. requirement is part of a larger global shift toward digital border controls. Australia and the United States 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 Authorization System (ETIAS), soon.

What makes the U.K.’s ETA different is the requirement that even connecting fliers secure one.

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

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

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

A U.K. 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.”