Boris Johnson is to face a vote of no confidence on Monday evening after the threshold of 54 letters from Conservative MPs seeking his departure was reached.

In a statement Graham Brady, the chair of the 1922 Committee that represents backbench Tories, said the threshold of 15% of MPs seeking a confidence vote, numbering 54, “has been exceeded”.

“In accordance with the rules, a ballot will be held between 1800 and 2000 today, Monday 6 June,” it continued. “The votes will be counted immediately afterwards. An announcement will be made at a time to be advised. Arrangements for the announcement will be released later today.”

To stay in office, Johnson needs to win the support of at least 50% of all Tory MPs plus one, totalling 180. If he does win he is theoretically safe from such a challenge for a year – although the rules can be changed.

Asked about this on Monday, Brady confirmed to reporters that this was the case: “Technically it’s possible for rules to be changed but the rule at present is there would be a period of grace.”

Brady said he told Johnson on Sunday that a vote would happen, and liaised over the timing: “He shared my view, which is also in line with the rules that we have in place, that that vote should happen as soon as it could reasonably take place, and that would be today.”

No 10 will hope the swift timetable means the largely disparate groups of rebels will be unable to coordinate or campaign.

A defence operation for Johnson has already swung into action, with whips sending Tory MPs a densely typed briefing note talking up his achievements while saying a leadership contest would create “a distracting, divisive and destructive civil war in the Conservative party”.

Johnson is due to address backbench MPs later in the day to try to rally support.

A No 10 spokesperson said: “Tonight is a chance to end months of speculation and allow the government to draw a line and move on, delivering on the people’s priorities.

“The PM welcomes the opportunity to make his case to MPs and will remind them that when they’re united and focused on the issues that matter to voters there is no more formidable political force.”

In an immediate show of support, the foreign secretary, Liz Truss – who is widely assumed to be a contender if Johnson goes – tweeted: “The prime minister has my 100% backing in today’s vote and I strongly encourage colleagues to support him … He has apologised for mistakes made. We must now focus on economic growth.”

Other cabinet ministers also issued supportive messages, among them Rishi Sunak, the chancellor, and Dominic Raab, the justice secretary and deputy prime minister.

Tory MPs opposed to Johnson were confident they had breached the 54-letter mark but remain uncertain as to whether they would win a confidence vote. It is held in person but as a secret ballot.

Sajid Javid, the health secretary, sent out on Monday’s broadcast round for the government, pledged Johnson would “stand and fight his corner” if he did face a confidence vote.

“If this threshold of 54 letters is reached there will be a confidence vote and in that case there should be. There may well be one,” Javid told Sky News before confirmation of the vote. “If there is, the prime minister will stand and fight his corner with a very, very strong case. So let’s just wait and see what happens.”

Just before the vote was announced, another former minister, Jesse Norman, released a condemnatory letter saying he had also sent a letter.

Johnson had “presided over a culture of casual law-breaking at 10 Downing Street in relation to Covid”, said Norman, a former Treasury minister. For Johnson to describe himself as vindicated by last month’s Sue Gray report was “grotesque”, he added.

Breaching Northern Ireland protocol “would be economically very damaging, politically foolhardy and almost certainly illegal”, Norman wrote, while the policy of deporting asylum seekers to Rwanda policy was “ugly, likely to be counterproductive, and of doubtful illegality”.

Johnson was showing no sense of purpose, Norman argued: “Rather, you are seeking simply to campaign, to keep changing the subject and to create political and cultural dividing lines mainly for your advantage.”

In the week and a half since the Gray report into No 10 parties was published, many Conservative MPs moved from not expecting a confidence vote against Johnson until the aftermath of two crucial byelections on 23 June, at the earliest, to becoming almost certain one would be held this week.

In the previous such vote, in December 2018, Theresa May won by 200 votes to 117, but having more than a third of her MPs oppose her forced May to set a timetable for her departure only five months later.