Jan 18 (Reuters) - 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 Britain 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. British media have reported that at least 11 gatherings took place at Johnson"s 10 Downing Street residence or in other government departments between May 2020 and April 2021, when COVID-19 rules limited how many people could meet socially. The scandals have seen approval ratings plummet for Johnson and the Conservative Party.