People are being urged to avoid using their second homes as calls mount for a crackdown on tourists and self-isolationists.
The Government said “essential travel” does not include visits to second homes, camp sites and caravan parks, and people should remain in their primary residence.
A police and crime commissioner in rural Wales has called for “lockdowns” to stop self-isolationists and tourists flooding in.
Dafydd Llewelyn described the Covid-19 epidemic as a “national emergency, not a national holiday” and demanded action.
Tourist boards across the UK have urged people to respect Government advice on social distancing and unnecessary travel.
Visit Cornwall asked holidaymakers online to “please stay away” to help slow the spread of the virus and avoid “needless lives being lost”.
Cherilyn Mackrory, Conservative MP for Truro and Falmouth, said: “My message to anyone considering travelling to Cornwall for a holiday at this time is simple – do not come.”
And Natural Resources Wales said it is considering “drastic measures” such as shutting car parks and trails.
Plaid Cymru leader Adam Price has written to Wales First Minister Mark Drakeford calling on him to close holiday accommodation, such as caravan parks, and restrict the use of second homes.
Scotland’s Tourism Secretary, Fiona Hyslop, also urged people travelling to remote areas of Scotland to “stay where you are”.
Meanwhile, Judy Murray, mother of tennis stars Andy and Jamie Murray, tweeted a simple message to “those relocating to the countryside” – a picture of a car and trailer with “Go home idiots” and “Covid-19” painted on the side.
Bridgwater MP Ian Liddell-Grainger has told tourists to stay out of Somerset and appealed to second-home owners with properties in areas such as Exmoor to keep away.
“Some caravan parks may have their own shops but a lot do not, which is going to mean more pressure on local shops which are already struggling to keep their regular customers supplied and more people out in the streets when we should be limiting public movement,” the Conservative MP said.
“And second-home owners who decide the country is a safer place to be at the moment are going to have precisely the same impact.”
In updated guidance on non-essential travel, the Department for Digital, Culture, Media & Sport said: “This guidance is for people planning to visit second homes or holiday premises during the coronavirus pandemic.
“Essential travel does not include visits to second homes, camp sites, caravan parks or similar, whether for isolation purposes or holidays.
“People should remain in their primary residence. Not taking these steps puts additional pressure on communities and services that are already at risk.”
Kim Conchie, chief executive of Cornwall Chamber of Commerce, said the effective ban on visiting second homes is “a really tricky” issue.
Mr Conchie told the PA news agency: “They’re second homes, people’s property, so to tell someone they can’t visit their own home is pretty dire.
“I imagine many people will give two fingers to that, saying ‘I’m going to self-isolate in my second home’.
“I have some sympathy with that. There are still shops open – as long as they don’t buy all their groceries from out of town, we do want that revenue here.
“It’s a difficult conundrum.
“Obviously you wouldn’t be very popular if you brought it down here and had to go to the Royal Cornwall Hospital and clutter up the facilities.
“The original message of ‘Don’t come at the moment’ – certainly bringing the disease here – that is obviously important.”