The coronavirus “rule of six” has come into force in England with people now risking fines of up to £3,200 for breaking the new restrictions.
Anyone meeting in groups of more than six in both indoor and outdoor settings will now be breaking the law.
It comes amid a rapid increase in the number of Covid-19 infections with more than 3,000 new cases being recorded in the UK for the third day in a row on Sunday – the first time since May that cases were above 3,000 on three consecutive days.
Regulations enabling police to enforce the new rule were published late on Sunday night, around 30 minutes before they came into effect.
Aside from limited exemptions including work and education, police will be able to disperse gatherings that breach the rule.
They will also be able to slap individuals with £100 fines, doubling up to a maximum of £3,200 for repeat offenders. Those who host raves or large unlawful events will continue to risk £10,000 fines.
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Home Office officials said that those who refuse fines will be arrested and taken to court.
Home Secretary Priti Patel said the recent rise in cases “makes it clear that more needs to be done to stop the spread of this disease”.
However, Oxford scientists have said the "catastrophic" rule of six should be "binned", adding that there is no evidence to support it.
Professor Carl Heneghan accused Boris Johnson of acting too hastily by introducing draconian new measures that could have huge social consequences.
The leading scientist and director of the Centre for Evidence-Based Medicine at Oxford University, told the Telegraph: "The rule of six policy should be binned."
He added: "Last week's roll of the dice with the 'rule of six' could well be the policy that tips the British public over the edge, for it is a disturbing decision that has no scientific evidence to back it up and may well end up having major social consequences."
Meanwhile, a Government source told the newspaper that a national curfew is "the obvious next step" if cases continue to rise.
Robert Buckland, the Justice Secretary, also told Sky's Sophy Ridge on Sunday that curfews could be needed as young people forget about the importance social distancing rules.
He said: "I think there's an issue about social occasions and social events and particularly young people getting together and enjoying themselves sometimes a bit too much and forgetting the importance of the rules."
The rule applies across England and replaces the existing ban on participating in gatherings of more than 30 and the current guidance on allowing two households to meet indoors.
Deputy Assistant Commissioner Matt Twist, who leads the Metropolitan Police’s response to coronavirus, said officers will be deployed in every borough to patrol public spaces and respond swiftly to incidents where groups gather in large numbers.
“Where people just won’t listen, and are putting everyone at risk, we absolutely will take enforcement action,” he said.
Chair of the National Police Chiefs’ Council Martin Hewitt has urged the public to “stick to the limits”.
He said: “Preventing the spread of coronavirus is a shared effort and police are playing our part alongside Government, businesses, hospitality owners, local authorities and others.
“Officers are in their communities following our approach to engage, explain and encourage people to follow the rules. We will issue fines when people refuse to comply.
“The demands on the police service are now at similar levels to before the pandemic, which makes it crucially important that we all take personal responsibility, stick to the limits and prevent the spread of this deadly virus.”
Meanwhile, a survey suggests a second peak is the number one concern among medics who want to avoid a return to the “horror and tragedy” of the pandemic’s early days.
The British Medical Association (BMA) poll found that 86 per cent of more than 8,000 doctors and medical students in England said that a second peak was likely or very likely in the next six months.
Dr Chaand Nagpaul, BMA council chairman, said: “With daily cases still alarmingly high, and winter just around the corner, we are at a critical crossroads in the fight against this deadly virus.
“All efforts must be made to avoid a repeat of the horror and tragedy we all experienced earlier this year.”
Officials will be hoping the warm and sunny weather forecast for Monday does not encourage people to gather in groups in outdoor spaces.
Temperatures as high as 29C (84F) are expected in some parts, according to the Met Office.
The Government said that as of 9am on Sunday, there had been a further 3,330 lab-confirmed cases of coronavirus in the UK.
Tough new Covid-19 lockdown measures were announced for parts of the UK on Friday as cases continued to rise and as the R number – the reproduction number of coronavirus transmission – climbed above one.
According to Government advisers, the last time R was above one was in early March.