As of February 25, 2026, U.S. citizens are required to have an Electronic Travel Authorisation (ETA) to travel to or through the United Kingdom. This is a new mandatory digital permission for visitors from "non-visa" countries (like the USA, Canada, and the EU) who are coming for short stays of up to six months. The ETA is not a visa, but a pre-travel security screening similar to the U.S. ESTA. You must apply for it online or via the UK ETA app before you even arrive at the airport; without it, you will likely be denied boarding by the airline. The application process is generally quick—most people receive a decision in minutes—and the cost is £16 per person. Once approved, the ETA is electronically linked to your passport and is valid for two years for multiple entries, or until your passport expires, whichever comes first. It is important to note that you need an ETA even if you are just "transiting" through a UK airport to a different destination. While the UK had a "grace period" for the rollout throughout 2025, enforcement is now strict, and travelers are advised to apply at least three working days before their departure to avoid any last-minute issues.