Boris Johnson has been admitted to hospital, 10 days after first testing positive for COVID-19, BBC News reports.
He's reportedly undergoing further testing after continuing to suffer symptoms of the deadly virus, which has killed nearly 70,000 people worldwide.
His doctor called it a "precautionary step".
His partner Carrie Symonds, pregnant with their first child together, has also shown symptoms of COVID-19 but hasn't undergone tests.
"I haven't needed to be tested and, after seven days of rest, I feel stronger and I'm on the mend," Symonds said at the weekend.
"Being pregnant with COVID-19 is obviously worrying. To other pregnant women, please do read and follow the most up to date guidance which I found to be very reassuring."
Johnson, 55, started working from home once discovering he had the disease last week, saying he had "mild symptoms".
"But be in no doubt that I can continue, thanks to the wizardry of modern technology, to communicate with all my top team to lead the national fightback against coronavirus," he said, urging Britons to stay home as much as possible.
The UK recorded its largest 24-hour increase in deaths from COVID-19 on Saturday (local time) - 621. The total death toll there is now 4934.