From January 20
As of January 20, most UK travellers heading to the US will need to get approval before flying.
The Electronic System for Travel Authorisation (ESTA) scheme means passengers must apply online to US Customs and Border Protection before heading to the airport.
The process takes around 15 minutes and should be done at least 72 hours before travel. Once granted, ESTA is valid for two years and allows multiple visits without the need to reapply.
Travellers can apply for an ESTA even if they have no firm plans to travel to the US.
Passengers without approval will not be able to fly, warns Silla Maizey, director of customer service at British Airways.
"It only takes 15 minutes to complete an ESTA and the vast majority of applicants will receive approval for travel within minutes, although it can take up to 72 hours," she said.
"Under the new regulations, we will not be able to accept US-bound customers for their flight if they have not received ESTA approval or if they do not hold a valid visa or Green Card."
ESTA was introduced in August 2008, and people already registered need not reapply until two years after their original application.
To apply, visit https://esta.cbp.dhs.gov. The service is currently free, but the US government admits it may introduce a fee in the future.
Countries in the visa waiver programme, who will now need to get approval using ESTA are: Andorra, Iceland, Norway, Australia, Ireland, Portugal, Austria, Italy, San Marino, Belgium, Japan, Singapore, Brunei, Latvia, Slovakia, Czech Republic, Liechtenstein, Slovenia, Denmark, Lithuania, South Korea, Estonia, Luxembourg, Spain, Finland, Malta, Sweden, France, Monaco, Switzerland, Germany, The Netherlands, United Kingdom, Hungary and New Zealand.