UK Prime Minister Boris Johnson cancelled a trip to northern England Thursday after a relative came down with Covid, officials said, a day after he apologised over a lockdown-breaking party.

"The prime minister will no longer be visiting Lancashire today due to a family member testing positive for coronavirus," a Downing Street spokesperson said.

"He will follow the guidance for vaccinated close contacts, including daily testing and limiting contact with others."

In Britain, fully vaccinated people no longer have to self-isolate after coming into contact with an infected person. But they are advised to "limit close contact with other people outside your household".

Johnson lives in a Downing Street flat with his wife Carrie and their two small children. 

For his critics, Johnson's scrupulous adherence to the rules now will contrast with his attendance at a party in the Downing Street garden in May 2020, when Britain was under a strict lockdown.

After days of stonewalling the scandal, on Wednesday he issued "heartfelt apologies", conceding that millions had observed the lockdown even at the cost of missing final farewells to dying relatives.

But opposition parties and even some Conservatives are demanding Johnson resign.