UK Prime Minister Boris Johnson visits a COVID-19 vaccination center on December 29, 2021 in Milton Keynes, England. Some 45.2 percent of the UK population have received either a third vaccine or booster shot as of Christmas week. The UK reported more than 128,600 COVID-19 infections on December 28. The country has recorded over 12.3 million cases in total.

British Prime Minister Boris Johnson is on the brink of facing a leadership challenge, according to reports, after an angry backlash over claims parties were held at his residence during coronavirus lockdowns.

After Johnson denied an accusation by his former adviser that he had lied to parliament about one party, the Daily Telegraph and ITV News, citing sources, said the required number of letters from his own lawmakers calling for a no-confidence vote in his leadership could be reached on Wednesday.

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.

Johnson's office did not respond to a request for comment.

Just two years ago, Johnson was the darling of his party after he secured the biggest Conservative majority since Margaret Thatcher in 1987, allowing him to deliver on promises to finally steer the UK out of the European Union.

But the relationship is rapidly souring under the impact of a steady stream of revelations about Downing Street's apparent flouting of strict lockdown rules.

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

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 in December 2021 as the Omicron strain spread rapidly across the UK. They included guidance to work from home where possible, masks for indoor settings and vaccine passports for mass events.

The removal of the restrictions would please many in his party who want to return to something akin to normal life as Johnson seeks to contain a rebellion triggered by the claims of rule-breaking parties.