People have been warned they are not allowed to drive from England into Wales for exercise as the two countries move to different lockdown rules.
Rules have been relaxed in England, meaning people can “drive to other destinations” for a walk.
The Welsh Government’s counsel general, Jeremy Miles, said rules “do not permit people to get in their car and drive to destinations in Wales”.
“And that also means people getting in their cars in England,” he added, saying police in Wales “absolutely have the power to fine” people for making non-essential journeys and not exercising near their homes.
South Wales Police Federation chairman, Steve Treharne, said where there had been an easing of regulations in England “people will see the messages and may take up more activities in Wales.”
Driving from England into Wales “still becomes an offence,” he said.
Gwent Police and Crime Commissioner, Jeff Cuthbert, feared the prime minister’s message would cause confusion: “I heard what Boris Johnson said yesterday, whilst we are clear what the rules in Wales are, certainly in the eyes of the public I’m sure what he has said has muddied the water.
“I worried about my colleagues in England and how they will police the idea of ‘staying alert’ – I’m not sure how that is going to happen.
“Undoubtedly, because of the strength of the English media there is going to be confusion in the minds of many people.
“It could well be because the prime minister didn’t mention the situation in Wales.”
North Wales Police and Crime Commissioner, Arfon Jones, dubbed differences in English and Welsh measures a “complete mess.”
“How are we supposed to get the message across to people living in England?” he said.
“It’s hard enough ensuring that those living in Wales are aware of the Welsh policy, because so many tune in to television channels from across the border.
“It’s going to be very difficult to get the message across that they should not come to Wales because the measures are different, and that they will be stopped and fined if they break these rules.”
Mr Johnson had made North Wales Police’s work “so much harder, and unnecessarily so”, he added.
Addressing the public on Sunday, Mr Johnson said people could “drive to other destinations”.
But the Welsh Government has stood by its “stay at home” message and is only allowing people to make essential journeys.
Among businesses allowed to open in England are golf courses.
Llanymynech Golf Club, in Shropshire, sits on the border and has 15 holes in Wales and three in England.
Wales Golf said courses in Wales “must remain closed for now.”
A spokesman said: “As a sport we must work together to resume play responsibly, as and when the relevant government determines it is safe to do so.
“We must ensure that the safety and wellbeing of everyone involved from golfers, to club staff and greenkeepers is maintained at all times.”