The public is being urged to “think twice” before heading to England’s beaches and country parks this weekend, despite the easing of lockdown rules.
Councils in beauty spots fear a surge in visitors could result in a rise in coronavirus infections.
In London, police have warned against taking part in “spontaneous or planned mass gatherings”.
There is no longer a limit on the amount of exercise allowed – or how far people can travel for it – in England.
This weekend will be the first since the rules changed so that people can spend more time outdoors “for leisure purposes”, including sunbathing.
But people in England should not travel to Wales, Scotland or Northern Ireland, where the public is still being told to avoid any travel which is not essential.
An estimated 15 million leisure trips will be made by car in the UK this weekend, an RAC survey suggests.
However, almost half of the journeys will be no more than 10 miles long, according to the motoring organisation’s poll of 1,317 drivers.
With sunny weather forecast in parts of the country this weekend, the County Councils Network is urging people to stay local.
The network, which represents 36 county authorities, warns that “day-trippers” who travel from towns and cities to exercise are likely to face long queues of traffic and difficulties parking.
And it cautions that country parks that reopened after lockdown rules were eased on Wednesday may be forced to close again if social distancing becomes impossible.
Julian German, the network’s rural spokesperson and leader of Cornwall Council, said England’s coastal and rural areas “will be there when this is over”.
“We are asking households to bear with us and please do their bit over the coming weeks by exercising locally,” he said.
“While councils will be allowing cars access to country parks, it does not change the unique situation of the need to maintain social distancing.”
He added that the councils wanted to prevent a repeat of the “unprecedented numbers of visitors” to parks and coastal areas over the weekend before lockdown was introduced in March.
Peter Williamson, chair of Norfolk and Suffolk Tourist Attractions Association, urged people not to visit the area, stressing that attractions, car parks and other facilities would be closed.
“What we’re trying to say to people is we’re not open, please don’t come because there is nothing for you here at this moment in time,” he told BBC Radio 5 Live.
The chief executive of the Lake District National Park has also asked people not to travel “because of the impact you will have on the local communities”.
Meanwhile, the Metropolitan Police Service stressed that “games of football… outdoor concerts or parties, protest, marches or assemblies are still not permitted”.
Officers “will engage and encourage people to comply” with public health regulations, the force added.
The Chief Constable of North Wales Police, Carl Foulkes, stressed the rules were different in Wales – where people should only be exercising from their home address – to those in England.
He told BBC Breakfast officers would be carrying out high visibility controls in key hotspots such as national parks and beaches, as well as road checks to ensure people were complying with the regulations.
Mr Foulkes said vehicles breaking the rules would be told to turn around, with officers using enforcement if necessary.
The warnings come as government scientific advisers say the infection rate in the UK has gone up – and is close to the point where the virus starts spreading rapidly.
The R-number – which represents the average number of people each infected person passes the virus on to – had been sitting between 0.5 and 0.9, but is now between 0.7 and 1.0.
It needs to be kept below one in order to stay in control.
Meanwhile, modelling published by the University of Cambridge and backed by Public Health England, suggests that while London has made the most progress with suppressing the virus, it is proving more stubborn in other parts of England.
The figures do not perfectly match those from the Sage group of government scientific advisers because it assesses multiple models to reach its conclusions.