Visitors to the UK will need an Electronic Travel Authorisation (ETA), a digital permission to travel that costs Stg 16 ($A31), first introduced in 2023 but not strictly enforced.
From Wednesday, it is mandatory for visitors to the UK to have obtained an ETA to travel and those without one will not be able to board their flight, ferry or train.
British and Irish citizens, including dual nationals, are exempt from the ETA requirement, which covers visitors from 85 countries including Australia.
However, the policy change means dual nationals - who could previously enter the UK using their non-British passport - will need a British passport or to pay Stg 589 ($A1128) for a certificate of entitlement.
"The ETA scheme is a vital part of our work to strengthen the UK's border security, helping to deliver a more efficient and modern service that works for both visitors and the British public," UK Home Office minister Mike Tapp said.
"I'd urge anyone wanting to travel to the UK to ensure they are travel-ready and have the right permission, to make their journey much smoother."
Some dual nationals have said enforcement of ETAs is being brought in at short notice and was poorly communicated.
There are fears holidays or work trips could be affected if people cannot get a passport or certificate of entitlement in time.
The Home Office previously said public information advising dual nationals to carry the correct documentation has been available since October 2024, and a "substantive communications campaign" has been running since 2023.
There are plans to increase the cost of an ETA to Stg 20 ($A38) at an unspecified future date.
The ETA permits multiple journeys to the UK for stays of up to six months over two years or until the holder's passport expires, whichever is sooner.
With Reuters
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