Boris Johnson has cancelled a planned visit to a vaccination centre after a member of his family tested positive for COVID-19.
An official statement said: "The Prime Minister will no longer be visiting Lancashire today due to a family member testing positive for coronavirus.
"He will follow the guidance for vaccinated close contacts, including daily testing and limiting contact with others.”
It is understood he will not be travelling anywhere and will not be giving a scheduled interview with ITV, but is not isolating.
The cancellation comes after the prime minister admitted attending a party in the gardens of Downing Street during the height of the first lockdown, when social gatherings were banned under COVID legislation.
Thursday's visit would have been the first public outing since he gave a humiliating apology in the Commons over the party, which was held on May 20, 2020.
Despite a number Cabinet ministers rallying to defend him, Johnson is facing increasing calls from within his own party to resign as prime minister.
It is thought backbench MPs are mounting a rebellion against their under-fire PM, and rumours of a vote of no confidence are gaining steam.
In order for a vote of no confidence to be called, 15% of Tory MPs — in this case 54 of them — must write to the chair of the 1922 Committee saying they have lost confidence in Johnson as their leader.
Calls for his resignation have continued to grow both in and out of the Conservative party - with Douglas Ross, leader of the Scottish Tories, becoming the most high-profile Tory to call for Johnson's resignation.
However, in symbol of how divisive the Number 10 party has been been, senior Tory and Commons leader Jacob Rees-Mogg lambasted Ross on Wednesday night over his decision.
"I don’t think Douglas Ross is a big figure," said Mogg when asked how significant Ross' intervention was, in comments unlikely to go down well in Scotland.
He added: "He has been constantly in opposition to the prime minister, he opposed Brexit, he is not somebody who you would ever expect to say helpful things about the prime minister."
Other Cabinet members have jumped in to defend their boss, although notably late to leap in were both the foreign secretary Liz Truss and chancellor Rishi Sunak.
Both are tipped as potential leadership contenders if the PM should fall, and their reluctance to throw their weight behind him has done little to instil confidence in his future.
Sunak had notably spent the day away from London on a visit in Devon instead of at Johnson's side as he issued his grovelling apology.