People must follow social distancing guidance while enjoying the sun, or Covid-19 cases “will rise again”, the UK’s chief medical adviser has warned.
Prof Chris Whitty’s remarks on social media came after a major incident was declared in Bournemouth when thousands of people flocked to the Dorset coast.
“Naturally people will want to enjoy the sun but we need to do so in a way that is safe for all,” he said.
The UK’s coronavirus death toll is now 43,320, a rise of 149 since Wednesday.
The latest figures, released by the Department for Health and Social Care, showed 307,980 people have tested positive across the UK.
Bournemouth, Christchurch and Poole Council said Bournemouth Beach was “stretched to the absolute hilt” on Thursday, while Dorset Police said there were reports of gridlocked roads, fights and overnight camping.
Council leader Vikki Slade said they were “absolutely appalled at the scenes witnessed on our beaches”.
Later, on Twitter, Prof Whitty said: “Covid-19 has gone down due to the efforts of everyone but is still in general circulation.
“If we do not follow social distancing guidance then cases will rise again.”
On Thursday, for the second consecutive day, the UK recorded its hottest temperature of the year so far, with highs of 33.3C (92F) at Heathrow Airport.
Both Wales and Scotland also individually recorded their hottest days of the year.
In Wales, the temperature reached 30.7C at Gogerddan, near Aberystwyth, beating the previous high of 30C from Wednesday. And in Scotland, a high of 30C was recorded in Prestwick, Gannett.
In Northern Ireland, a high of 25.5C was recorded in Aldegrove.
An amber level three heat-health alert, issued by the Met Office, was extended on Thursday to take in Yorkshire and the east and south of England as well as the West and East Midlands.
That means people should drink plenty of fluids, avoid consuming excess alcohol and “look out for” young children, babies and those with underlying health conditions, the Met Office said.
Commenting on the crowded scenes in his constituency, Bournemouth East MP Tobias Ellwood said he had asked the government to dispatch additional officers to Dorset if needed to deal with traffic and antisocial behaviour.
“It is very sad to see a number of people being selfish and also acting dangerously,” he said.
Mr Ellwood said it was “not practical” to close Dorset’s beaches altogether but suggested signs warning about overcrowding could be put up at railway stations and on approaching motorways.
He added the government needed to be “dynamic” in its response to beach crowding, otherwise the lockdown would have “been for nothing”.