Americans now need an ETA to visit the UK. What it is, how to get one

A passport alone will no longer get you into the United Kingdom.

As of late February, the British government started requiring an electronic travel authorization (ETA) or visa for most foreign tourists.

Starting April 23, an ETA will also be required to visit the Channel islands of Jersey and Guernsey off the coast of France and the Isle of Man in the Irish Sea.

"Visitors without an ETA will not be able to board their transport and cannot travel to the UK, unless exempt," the U.K.'s Home Office, or interior department, posted on its official blog.

Here's what travelers should know if they want to visit.