From 12 January 2009, if you are travelling under the Visa Waiver Programme, you must have Travel Authorisation. You can receive Travel Authorisation by completing an online form.

The application process may take up to 15-20 minutes and authorisation is normally granted immediately, but may take up to 72 hours.

We recommend that you apply for authorisation at the time of booking or at least 72 hours before your flight, to ensure that you are able to travel. Those making late bookings can still apply, but there is a risk that they may not receive immediate confirmation.

Travel Authorisation will become compulsory in January 2009, but you can now apply voluntarily. It is recommended that you do so to familiarise yourself with the new system.

Once granted, Travel Authorisation is valid for multiple entries to the US for two years, unless any answers to the YES/NO questions on the form change or a new passport is required.

You are also required to provide Advance Passenger Information to the US government each time you travel to the US.

If you have a resident permit 'green card', or are travelling on a visa, you will not need to apply for authorisation as you are not travelling under the Visa Waiver Programme.

Click here for link to the ESTA site