Australians heading to Britain face tougher travel rules from Wednesday – prompting a warning for travellers.

Under updated entry requirements from January 8, millions of travellers heading to Britain for less than six months require an Electronic Travel Authorisation to enter the country.

The rules also apply to Australian passport holders, who have previously enjoyed visa-free travel. They will be among those required to apply for the online ETA, which costs £10 ($19.99).

The scheme is similar to the Electronic System for Travel Authorisation used by the United States – with one significant catch.

The US program issues confirmation of entry approval within just minutes, while the new British scheme can take days.

“You’ll usually get a decision within three working days, but you may get a quicker decision,” the British government said in announcing the program’s expansion last year.

“Occasionally, it may take longer than three working days.”

The change, which also affects millions of other travellers, including those from nations such as New Zealand, Canada, the United States and Japan, means that airline staff will check that passengers on flights to Britain have an approved ETA.

But anyone who turns up at the airport without one cannot expect to fix the omission with a quick application on their smartphone for instant approval. The mix-up could mean missed flights and costly new tickets – far from the ideal way to start a holiday.

The ETA scheme is part of Britain’s push to digitise its border security. It will eventually require anyone who does not need a visa and was not born in the UK or Ireland to seek permission to travel before arriving there.

In announcing the change, Britain’s migration minister Seema Malhotra said tourism was expected to contribute more than £32 billion ($62.8 billion) to the country’s economy.

“Digitisation enables a smooth experience for the millions of people who pass through the border every year,” she said.

“The worldwide expansion of the ETA demonstrates our commitment to enhancing security through new technology and embedding a modern immigration system.”

Australians travelling to Britain as tourists for less than six months normally don’t need a visa. They will still have to complete the new ETA form and pay the fee.

Australians with dual British or Irish citizenship are exempt from the ETA process if they use their British or Irish passport or another passport containing a certificate of entitlement.

With an ETA, a traveller cannot spend longer than six months in Britain, do paid or unpaid work there, claim public funds or benefits, spend extended time in successive visits or register to marry someone.

Travellers can apply for the ETA through the British government’s online portal or with the ETA app that is available on the Google Play and Apple stores.

Contact and passport details must be supplied, along with a valid ID. Applicants must also supply a photo.

The ETA lasts for two years (unless the passport expires earlier) and Australians can travel as many times as needed during that period, but will still be required to go through border security.