From 20 January 2010 ESTA (Electronic System for Travel Authorisation) is a mandatory enforcement for travel to the United States.

You will need to fill in the required information and it is advised that the application will need to be made at least 72 hours prior to travel. If passengers do not have an ESTA, they may not be issued with a boarding pass or clear passport/visa checks, and therefore may NOT be allowed to travel to the United States.

You are strongly advised to provide API (Advanced Passenger Information) prior to travel. This requirement applies to all international flights arriving in or departing from the United Kingdom* and for all passengers travelling to and from, or in transit to the following: Antigua and Barbuda, Australia, Barbados, Bermuda, Canada, China (except Hong Kong), Cuba, Dominica, Grenada, Guyana, Jamaica, Japan, Korea, Kuwait, Mexico, New Zealand, South Africa, Spain, St Kitts and Nevis, St Vincent and the Grenadines, Thailand, Trinidad and Tobago, United Arab Emirates and USA.

Read more on ESTA and API here .