People in England who refuse an order to self-isolate could be fined up to £10,000, the government has said.
The new legal duty requires people to self-isolate if they test positive for coronavirus, or are traced as a close contact, from 28 September.
New measures also include a one-off £500 support payment for those on lower incomes, and a penalty for employers who punish those told to self-isolate.
It comes as the PM considers tightening restrictions after a surge in cases.
A further 4,422 new Covid-19 cases and 27 deaths were reported on Saturday.
There were 350 new cases reported in Scotland, the highest daily increase since May, 212 new cases in Wales, and 222 in Northern Ireland.
Fines will initially start at £1,000 rising to £10,000 for repeat offenders, and for “the most egregious breaches”. Up until now, advice to self-isolate has been guidance only.
Prime Minister Boris Johnson said: “So nobody underestimates just how important this is, new regulations will mean you are legally obliged to do so if you have the virus or have been asked to do so by NHS Test and Trace. People who choose to ignore the rules will face significant fines.
“We need to do all we can to control the spread of this virus, to prevent the most vulnerable people from becoming infected, and to protect the NHS and save lives.”
Health Secretary Matt Hancock said the nation was at a “tipping point” and warned more measures would be brought in unless everyone followed the rules.
“I don’t want to see more measures but unfortunately if people don’t follow the rules that’s how the virus spreads,” he told Sky News.
Asked if he would report anyone he knew breaking the rules, he said: “Yes. And everybody should.”
More than 19,000 fines have been issued in England and Wales for alleged breaches of coronavirus laws, the attorney general said earlier this week, but more than half have not been paid so far.
Labour leader Sir Keir Starmer told Sky News he supported the new fines, saying “a small number of people are breaking the rules and something has to be done about that”.
But he warned it was not a “silver bullet” while the testing system was “barely serviceable”.
