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.
Announcing the new rules for England, Prime Minister Boris Johnson said the best way to fight the virus was for everyone to follow the rules.
“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”, he said.