LONDON, June 5 (Reuters) - Officials in British prime minister Boris Johnson's ruling Conservative Party expect a challenge to his leadership this week and have pencilled in a vote for Wednesday, the Sunday Times newspaper reported.

A growing number of Conservative lawmakers have said they have lost faith in Johnson's government over a "partygate" scandal, with some saying they have submitted letters to officially call for a vote of confidence in their leader.

At least 54 Conservative members of parliament are required to formally request a confidence vote to the chairman of the party's 1922 Committee for one to be triggered. The letters are confidential so only the chairman of the committee knows how many have been submitted.

More than 25 lawmakers have gone public with their letters so far and the Times said party officials and rebel lawmakers believed they were on the verge of the 54 threshold, with one believing the key number had already been passed.

"Officers of the 1922 executive have already pencilled in Wednesday as the day for the leadership vote," the newspaper reported.

Johnson has repeatedly apologised for his conduct after an official report found both he and Downing Street officials broke stringent laws that his government made during the pandemic, holding alcohol-fuelled gatherings at the height of lockdowns.

He was jeered by the public when he arrived at a service of thanksgiving for Queen Elizabeth on Friday.

Johnson has said he will not resign because there are too many challenges facing the government and it would not be responsible to walk away.

(Reporting by Kate Holton; Editing by Catherine Evans)