People in parts of the UK with high rates of Covid-19 will be banned from travelling to Wales under plans announced by Wales' first minister.
Mark Drakeford said he would go ahead if the prime minister did not impose travel restrictions in England. It is expected the ban will cover all of Northern Ireland, England's tier two and three areas and the Scottish central belt.
The UK government said the decision was "disappointing". And the head of the Police Federation in Wales warned the rules could be "unenforceable".
"There will also be plenty of individuals travelling legitimately from areas which are not high risk, and this will only add to the other difficulties officers face when policing the existing regulations," Mark Bleasdale said.
Scotland's First Minister Nicola Sturgeon said she fully supported Mr Drakeford's move. She said she would write to the prime minister seeking urgent talks over UK-wide travel restrictions and called for a "sensible agreement" between the four nations.
Mr Drakeford had written twice in recent weeks to Boris Johnson asking for travel to be restricted in and out of areas with high levels of transmission in England.
The UK government has repeatedly refused to do so, instead asking people in the Liverpool City Region to avoid non-essential travel.
UK government ministers plan to write to Mr Drakeford to raise their concerns about the travel ban and some of the language being used. Mr Drakeford said: "Evidence from public health professionals suggests coronavirus is moving from east to west across the UK and across Wales.
"As a general rule, it is concentrating in urban areas and then spreading to more sparsely populated areas as a result of people travelling."
He said the law will be "essentially designed to prevent people going to those holiday parts of Wales, the far west and the far southwest of Wales, where the virus is still in very low circulation".
The rules are planned to come into force on Friday at 18:00 BST.
Travel to and from the 17 local lockdown areas in Wales - covering most of the population - is already restricted. People can already only travel to places like Cardiff or Swansea if they have a reasonable excuse like work or education.
But travel is possible between areas that are subject to restrictions in England and the parts of Wales not under lockdown rules, such as Powys and Pembrokeshire.
The Welsh Government's plans would effectively restrict travel to those places, and also cover anyone from a low-Covid area of Wales travelling to high-Covid areas elsewhere who then return to Wales. Similar exemptions will apply to those that exist for Welsh local lockdown areas. Enforcement details are not been confirmed but it is likely breaking the law could attract a fixed penalty notice, or a fine if it goes to court.
A Welsh Government source stressed fines would be a last resort.
Prime Minister Boris Johnson says people should use their common sense when travelling around the UK.
The first minister said it was "absolutely possible" to enforce the ban, "we did exactly this for many many weeks earlier this year".
He warned anyone planning to "evade" a police officer "trying to prevent people from travelling onwards".
"When they arrive in the far west of Wales, I'm afraid they will meet a local population that are fearful, that are anxious and are on the lookout for people who shouldn't be in those areas.
"So your difficulties aren't over by evading the police, there'll be other checks in the system."