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.
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.
The chief executive of the Lake District National Park has also asked people not to travel to the area “because of the impact you will have on the local communities”.
Richard Leafe told BBC Radio 4’s Today programme that for those who “insist on coming this weekend”, there will be a system for assessing how full car parks are.
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 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.