Britons have been accused of "ignoring" restrictions during the sunny bank holiday weekend, as the police and coastguard criticised those failing to observe guidelines.
HM Coastguard said it had the highest number of call-outs since the lockdown began on Friday, with 97 incidents - a 54% increase compared with April's daily average.
Meanwhile in London, police admitted they were struggling to stop people gathering in parks amid the fine weather.
A Hackney Police statement, posted on its official Twitter page alongside an image believed to be of London Fields, said: "Sadly we're fighting a losing battle in the parks today.
"Literally hundreds of people sitting having pizza, beers, wines. As always a big thank you to those that are observing the guidelines."
COVID-19 rule-breakers are being warned that they are putting emergency workers at risk, and putting avoidable pressure on the NHS.
Temperatures are set to reach 26C (78.8F) in parts of the UK on Saturday in what is promising to be the hottest day of the year so far - with London and the South East expected to be hotter than Ibiza and St Tropez.
This has fuelled fears that the government's stay-at-home advice will not be followed, with Britons flocking to parks and beaches for days out.
HM Coastguard commander Matt Leat said: "People are ignoring the measures put into place by the government.
"I completely understand that the weather and the bank holiday coupled with the fact that we've been in this lockdown situation for just over six weeks has tempted people out to our beautiful coasts.
"However, as the government said only yesterday, it's really vital that we all continue to observe the guidance.
"Every time we get a 999 or distress call, we will always respond but the minute we send in a rescue response, we're putting our frontline responders at risk as well as putting the NHS under avoidable pressure.
"Please, please continue to observe the #StayHomeSaveLives message - it's still in place for a reason. Exercise locally and stretch your legs, not our resources."
The prime minister's Twitter account is urging the public to "keep going for everyone's sake" - stressing that their sacrifice is making a difference.
Meanwhile, Scotland's top police officer has warned that policing will be more challenging if England and Scotland take different paths on lockdown.
Cautioning against any divergence, Police Scotland Chief Constable Iain Livingstone told BBC Radio 4's Today programme: "I think it would make the consistency of public messaging harder, people would be hearing different things when they listen to different forms of media.
"It would be harder than it's been but I'm confident that the police would be able to respond to any differentials that the politicians decide upon."
He also said policing the next phase of the coronavirus will be more challenging generally.
Mr Livingstone said: "Any easement or any change around that that has a level of distinction, whether that's by locality, whether that's by sector, whether that's by criteria, age or occupation, inevitably adds a level of complexity."
The Westminster government has spoken of a "four nations approach" to tackling the crisis, but there have been signs of tensions between Downing Street and the devolved administrations.
Scotland's First Minister Nicola Sturgeon recently warned she would not be "pressured" into "prematurely" easing her country's COVID-19 restrictions and urged Scots to stick with the current advice.
It follows concerns over "mixed messages" to the public, with reports Boris Johnson could allow sunbathing and picnics to be permitted in England from as early as Monday.
Ministers have since downplayed this, insisting there would be no "dramatic overnight change" of the lockdown rules.