Australians are being warned entry requirements for the UK are about to change within hours.

From January 8, the millions of travellers visiting the UK for less than six months will require an Electronic Travel Authorisation (ETA) to enter.

Australians who have previously enjoyed visa-free travel will now be required to apply online for an ETA, costing about $20, or £10.

Know the news with the 7NEWS app: Download today

Travellers have been able to apply for an ETA since November 27 through the UK ETA app or the UK government website.

“Australians visiting the UK, who do not need a visa for short stays or who do not have any other UK immigration status, will need an ETA to travel to or transit through the UK,” the Australian government’s Smartraveller posted online last year.

“An ETA permits multiple visits to the UK for stays of up to six months at a time over two years or until the holder’s passport expires, whichever is sooner.”

The UK government states that everyone travelling to the UK will now need to purchase an ETA, including babies and children, with no refunds to be offered after the application.

ETAs will be processed and a decision made within three working days. However, the UK government has warned it may take longer depending on demand.

The authorisation will then be valid for two years, or until the holder’s passport expires.

A photo of the applicant’s passport, as well as a photo of their face, will be required when applying.

Australians with dual British or Irish citizenship do not need an ETA if they are travelling using their British passport, Irish passport or other passport containing a certificate of entitlement.

The UK is the fifth most-visited destination for Australian travellers, according to data released by the Australian Bureau of Statistics in August.

The only four countries more frequently visited were Indonesia, New Zealand, the USA and Japan.