British Prime Minister Boris Johnson has warned Britons in a letter delivered to 30 million households that things will get worse before they get better, as he himself self-isolates in Downing Street to recover from a coronavirus infection.

The U.K. has reported 17,089 confirmed cases of the disease and 1,019 deaths, with the peak of the epidemic in the country expected to arrive in a matter of weeks.

In the letter to be delivered to homes, Johnson urges people to stick with the lockdown measures that his government has imposed to try to prevent the state National Health Service from being overwhelmed by a surge of cases.

"We know things will get worse before they get better," Johnson writes. "At this moment of national emergency, I urge you, please, to stay at home, protect the NHS and save lives."

Senior minister Michael Gove said on Sunday the government was "very concerned" about the death toll. He also said the country had boosted the amount of testing for the virus.

"The number of tests being carried out has hit 10,000 a day. We want to increase that to 25,000 a day."