Mr Johnson has worked from home since it was announced that he had tested positive for coronavirus on 27 March.
Johnson will be assessed by medics in what is being described as a "precautionary step" by 10 Downing Street, according to widespread reports.
Britain's main opposition Labour Party will elect a new leader on Saturday, with Brexit policy chief Keir Starmer the frontrunner to take over a deeply divided party.
He had been scheduled to re-emerge on Friday after a week of recovery and working remotely, but said he would remain at home because he still had a high temperature - one of the symptoms.
(Alliance News) - People in the United Kingdom have been urged to stay at home this weekend, despite the warm weather, to prevent the spread of coronavirus, as a key government adviser warned that the lockdown measures are merely a "placeholder".
In his message, Johnson warned people across the nation, not to be tempted to venture outside at the weekend, with the better, warmer weather forecast.
In his victory speech, posted online, Sir Keir said Labour had a "shared purpose" with the Government in getting the country through the Covid-19 outbreak.
Jeremy Hunt, the former Health Secretary who has been critical of the government response, said: "Thoughts with Boris Johnson this evening".
In order to meet the demand, the government said it would work with private firms such as Amazon and chemist Boots, and that three new "mega labs" would soon be online.
Johnny Mercer, the Conservative MP for Plymouth, Moor View, tweeted a picture of himself with the Prime Minister with the message "Get well soon mate" followed by a red heart. We hope that we have succeeded by giving you the full details and information.