A new system of checks on US-bound air passengers from the UK has come into force, with all travellers required to complete an online form before leaving.
The Electronic System for Travel Authorisation form, which takes up to 72 hours to be approved, gives flyers prior approval for entry to the US.
It replaces the green I-94 card which passengers have previously filled in on their flight into the US.
The scheme first came in a year ago but is now mandatory for all travellers.
The Esta form applies to all 35 countries with US "visa waiver" status.
An approved Esta is valid for two years and can be used for multiple entries into the US.
No form, no flight
Most people who fill in the form should receive approval from the US Department of Homeland Security within a few minutes.
But British Airways and American Airlines Major are advising travellers to apply at least 72 hours in advance.
Both airlines have said people without valid Esta forms will not be allowed to board flights to the US.
The scheme is currently free but this may change as the US Senate voted in September to charge visitors from visa waiver countries a minimum of $10 (£7) to fill in the form and enter the country.
The EU has criticised the Esta scheme as effectively a move to visas.
The scheme was introduced in January 2009 but is mandatory from Wednesday. Airlines had feared the new rules could cause chaos at British airports but say the phased introduction should have minimised problems.
About four million Britons visit the US each year.