Originally Published: 06 JUN 22 03:22 ET

Updated: 06 JUN 22 12:18 ET

By Ivana Kottasová, Rob Picheta and Luke McGee, CNN

(CNN) -- British Prime Minister Boris Johnson will face a vote of confidence on Monday, triggered by discontented lawmakers in his own party and following a series of scandals and months of speculation about his future.

Graham Brady, chairman of the 1922 Committee of backbench lawmakers, said in a statement Monday that the number of Conservative Party parliamentarians calling for the vote had reached the necessary threshold. The vote will be held between 6 p.m. and 8 p.m. local time on Monday, with results expected to be announced about an hour after that.

If 180 Conservative lawmakers -- a simple majority -- vote against Johnson, he will cease to be leader of the party and will be forced from office, less than three years after winning a general election in a landslide. Under Conservative Party rules, Johnson would not be allowed to run in the leadership contest following his ouster.

If Johnson wins the vote, he will remain both as leader of the ruling party and as Prime Minister.

Johnson's premiership has been shaken by the so-called "Partygate" scandal, with months of allegations of parties and gatherings at the heart of his government during various stages of pandemic lockdown eroding confidence in his leadership.

A damning report by senior civil servant Sue Gray, published late last month, found a culture of partying and socializing among Johnson's staff while millions of Britons were banned from seeing their friends and relatives. He has also been criticized for his response to a cost-of-living crisis.

Johnson addressed the 1922 Committee in person on Monday afternoon, just hours ahead of the vote. When he entered the room, he was greeted with loud desk-banging by loyalists wanting to express their support.

James Cleverly, one of the MPs in the room, later described Johnson as being "in serious mood, light on jokes, heavy on plans and policy."

According to PA Media, the Prime Minister wrote a letter to Conservative MPs earlier Monday urging them to support him and reminding them that he'd led the party to its biggest electoral win in 40 years.

Johnson said the confidence vote presented "the moment to draw a line under the issues our opponents want us to talk about -- and to focus instead on what really matters," according to PA.

While the vote is confidential, a number of Conservative MPs have publicly voiced their opposition to the Prime Minister.

Jeremy Hunt, who lost the 2019 Conservative leadership election to Johnson and is seen as a potential candidate to replace him, said he would vote against Johnson. Hunt is a well known figure in British politics, having served as the health secretary and the foreign secretary in the past.

"Having been trusted with power, Conservative MPs know in our hearts we are not giving the British people the leadership they deserve," Hunt wrote on Twitter. "We are not offering the integrity, competence and vision necessary to unleash the enormous potential of our country."

Another Conservative MP, Jesse Norman, told Johnson that his remaining in office "not only insults the electorate... it makes a decisive change of government at the next election much more likely."

Norman, who represents the Hereford and South Herefordshire constituency, released his letter of no confidence just moments before the vote was announced on Monday.

While he said the Prime Minister's response to the Sue Gray report was "grotesque," most of his letter focused on Johnson's other policies, including the government's new policy of sending some asylum seekers to Rwanda, which Norman called "ugly, likely to be counterproductive and of doubtful legality."

Conservative MP John Penrose quit his role as the UK government's anti-corruption tsar on Monday, claiming that Johnson had broken the government's ministerial code and quoting the Sue Gray report which highlighted "failures of leadership and judgment" inside Downing Street.

"I'm sorry to have to resign as the PM's Anti-Corruption Tsar but, after his reply last week about the Ministerial Code, it's pretty clear he has broken it. That's a resigning matter for me, and it should be for the PM too," Penrose said on his official Twitter profile.

Johnson's approval ratings have been plunging and there has been a growing sense among some parts of his ruling Conservative Party that he is becoming a liability. The party is facing two difficult parliamentary by-elections in late June after two of their backbenchers were forced to resign amid their own scandals.

Keir Starmer, the leader of the opposition Labour Party, has urged Conservative MPs to remove Johnson. Speaking to LBC radio, he said: "I think they've got to show some leadership and vote against the Prime Minister. He's lost the trust of the country, I think that's pretty clear on all the evidence I've seen."

In a sign of the public's displeasure, the Prime Minister was booed Friday by some members of public as he arrived at London's St Paul's Cathedral for a service of thanksgiving held as part of the Queen's Platinum Jubilee celebrations.

Johnson's backers have rushed to his defense in recent weeks, arguing that it is not the right time to trigger a leadership contest given the multitude of crises the country is facing -- including the war in Ukraine.

Several of Johnson's top ministers have already declared their support for him. UK Foreign Secretary Liz Truss said she was firmly behind Johnson. "The Prime Minister has my 100% backing in today's vote and I strongly encourage colleagues to support him," Truss posted on Twitter.

Chancellor Rishi Sunak also tweeted that he would back Johnson in the vote and "will continue to back him as we focus on growing the economy, tackling the cost of living and clearing the Covid backlogs."

Deputy Prime Minister Dominic Raab said the Conservative Party needed to back the Prime Minister, "unite and focus on delivering the people's priorities."

If Johnson were to lose Monday's vote, he would likely remain as prime minister until a new Conservative candidate was elected to lead the party; at that point, Johnson would inform the Queen of his intention to resign as prime minister and recommend that whoever won the leadership contest was invited to form a government.

Truss, Sunak and Raab are all considered possible contenders for the leadership, although their proximity to the Prime Minister might become a liability.

Analysts commenting on the impending vote said it was difficult to predict the outcome.

"The case against Johnson is clear cut. Following escalating scandals and a souring economy, Johnson's Conservatives are sliding badly in the polls ... On the other hand, Johnson is a proven election winner -- this counts for a lot," Kallum Pickering, senior economist at Berenberg bank, wrote in a note to clients.

In the last general election in 2019, under Johnson's leadership, the Conservative Party won its biggest majority since 1987.

Mujtaba Rahman, Eurasia Group's managing director for Europe, wrote in a note to clients: "If there is a sizeable vote against him -- say 100 MPs or more -- Johnson may be irretrievably damaged. He, and the public, will know that a sizeable number of his MPs do not support him."

If Johnson wins the vote comfortably, he could arguably emerge stronger within his party. Under current party rules -- which can be changed at any time -- he'd be immune from another leadership challenge for 12 months.

A narrow win, by contrast, would leave Johnson's reputation diminished even if it does not topple his government. Disappointing results in this month's by-elections could heap more pressure on Johnson ahead of a national general election expected in 2024.

A new snap poll of 2,062 British adults conducted on Monday by Opinium, a British market research agency, suggested that 59% of those surveyed think Conservative MPs should vote to remove Johnson from the office. The poll also indicated that 58% of voters think Johnson's leadership has been bad for the country, while 37% think it has been good.

However, a majority of Conservative voters surveyed said Conservative MPs should vote to keep the prime minister, with only 34% of Conservative voters wanting him to go, according to Opinium.

Starmer said the vote on Monday was the "beginning of the end."

"If you look at the previous examples of no-confidence votes even when Conservative prime ministers survive those, and he might survive it tonight, the damage is already done," he said.

Johnson's predecessor Theresa May was the last sitting British leader to face a no-confidence vote from their own party. May narrowly survived that vote, called amid months of chaos over her doomed Brexit deal, but ultimately resigned months later.

"Should he win, Johnson will still face the two by-elections and an investigation by the Commons Privileges Committee into whether he misled Parliament over the Partygate affair. This means his government will likely remain distracted and unstable," Rahman said.

Under Conservative Party rules, if MPs want to get rid of their leader, they submit a confidential letter of no confidence to the chair of the 1922 Committee, a group of backbench lawmakers who do not hold government posts. The process is murky -- the letters are kept secret and the chair, currently Brady, doesn't even reveal how many have been handed in.

When 15% of Conservative lawmakers have submitted letters, a vote of confidence is triggered among all Conservative lawmakers. The current makeup of the House of Commons means that at least 54 MPs have submitted letters of no confidence.

The scandal over parties is not the first to dent Johnson's reputation. He has been dogged by accusations that he accepted improper donations to fund a renovation of his Downing Street apartment, while his government has been accused of handing lucrative Covid-19 contracts to people with links to the Conservative Party. Johnson's spokesman has insisted he "acted in accordance with the rules at all times."