British Prime Minister Boris Johnson was on Wednesday fighting to shore up his premiership after a revolt by his own lawmakers who are angry over a series of lockdown parties in Downing Street.

Propelled into the top job to "get Brexit done", Johnson in 2019 won his party's biggest majority in more than 30 years but now faces calls to resign after a series of revelations about gatherings in Downing Street during Covid lockdowns.

Johnson has repeatedly apologised for the gatherings, and said that he didn't know about many of the events, though he attended what he said he thought was a work event on May 20, 2020.

To trigger a leadership challenge, 54 of the 360 Conservative MPs in parliament must write letters of no confidence to the chairman of the party's 1922 Committee.

As many as 20 Conservative lawmakers who won their seats at the last general election in 2019 plan to submit letters of no confidence in Johnson, the Telegraph reported.

"Group of 2019 MPs to submit letters to try to hit threshold of 54 to trigger a contest," BBC Political Editor Laura Kuenssberg said. "They might hit 54."

An analysis by The Times newspaper showed that 58 Conservative lawmakers had criticised the prime minister.

The letters are confidential, so the chairman is the only person who knows how many lawmakers have actually written them.

Johnson will address parliament on Wednesday after his Cabinet is expected to approve plans to end the recent restrictions imposed to tackle the spread of Covid-19 in England.

The "Plan B" measures were introduced by the government last month as the Omicron strain spread rapidly across Britain. They included guidance to work from home where possible, masks for indoor settings and vaccine passports for mass events.