Dame Cressida Dick, UK’s most senior police officer said that Covid rule-breakers were now increasingly likely to be fined by officers.
Writing in the Times, she said people were still holding house parties, raves and gambling gatherings in basements.
Health Secretary Matt Hancock has said tighter measures cannot be ruled out.
But speaking at a Downing Street press conference on Monday, Mr Hancock pleaded for people to follow the current rules, saying: “It is your actions now that can make a difference.”
The Scottish government’s cabinet is due to meet on Tuesday to discuss restrictions.
It comes as Downing Street insisted Boris Johnson did not breach lockdown restrictions when he went cycling in east London at the weekend. The PM was criticised for travelling seven miles to exercise, with government rules saying people should stay local.
The latest figures on Monday showed a further 529 people had died within 28 days of a positive test in the UK, while another 46,169 cases were reported.
There are also more than 32,200 people in hospital in the UK with coronavirus, data shows.
In her warning in the Times, Dame Cressida said officers were still finding people breaking the rules, despite clear laws that ban social gatherings.
“Most people are doing the right thing to keep each other safe, but sadly a small minority of people continue to flagrantly ignore the rules, for example by holding house parties, meeting in basements to gamble or breaking into railway arches for unlicensed raves,” she said.
“It is preposterous to me that anyone could be unaware of our duty to do all we can to stop the spread of the virus.
“We have been clear that those who breach Covid-19 legislation are increasingly likely to face fines.”
Dame Cressida said officers would now be moving “much more quickly to enforcement action”.
Police chiefs have been under increasing pressure to enforce the lockdown laws – with a number of news reports about breaches of Covid rules in recent days.
In England and Wales, police have issued 8,000 penalties since November.
Officers have been stopping people and asking them where they have travelled from, while elsewhere people have been discovered using pubs and gyms.
In one case, two women were fined £200 each by Derbyshire Police when they drove five miles for a walk together – but the force has since withdrawn the penalties.
After that incident gained widespread media attention, the National Police Chiefs’ Council issued fresh guidance to officers.
Home Secretary Priti Patel has defended the way police have handled breaches, saying there is a need for “strong enforcement”.
But psychologist Professor Stephen Reicher said evidence suggested 80-90% of people were by and large obeying the rules – except when it came to self-isolation where compliance was lower.
“The problem isn’t people breaking the rules,” said Mr Reicher, who sits on a committee of behavioural scientists that advises the Scientific Advisory Group for Emergencies.
“In many cases it’s either the rules aren’t clear enough, or the rules are too soft, or else people don’t have the support to do what they’re asked to do.”
Speaking on BBC Newsnight’s programme, he said one reason the virus was still spreading was because many people were still going to work. “We are defining all sorts of jobs as essential that aren’t,” he said.
Conservative former health minister Steve Brine led calls for an end to non-essential takeaway sales, including coffee.
“Stop the coffee,” he told Newsnight. “There are so many things that we are doing, which are allowed in the rules… but I just don’t think they are wise right now.”
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