Prime Minister Boris Johnson on Monday ordered Britons to stay at home to try to halt the spread of coronavirus, imposing curbs on everyday life without precedent in peacetime.

All but essential shops will close and people should no longer meet family or friends or risk being fined, Johnson said on Monday in a televised address to the nation.

Johnson had resisted pressure to impose a full lockdown even as other European countries had done so, but was forced to change tack as projections showed the health system could become overwhelmed.

Deaths from the virus in Britain jumped to 335 on Monday as the government said the military would help ship millions of items of personal protective equipment (PPE) including masks to healthcare workers who have complained of shortages.

"From this evening I must give the British people a very simple instruction - you must stay at home," Johnson said in a televised address, replacing his daily news conference.

They would only be allowed to leave their homes to shop for basic necessities, exercise, for a medical need, to provide care or traveling to and from work where absolutely necessary.

"That's all - these are the only reasons you should leave your home," he said, adding that people should not meet friends or family members who do not live in their home.

"If you don't follow the rules, the police will have the powers to enforce them, including through fines and dispersing gatherings," he warned.

The new measures would be reviewed in three weeks, and relaxed if possible.

The British Retail Consortium said shop owners understood the gravity of the situation, while former health secretary Jeremy Hunt said Johnson's decisions were "absolutely right."