The British Prime Minister, Boris Johnson, will face a test of Conservative Party loyalty on Thursday when MPs vote on whether he should be investigated for lying to them about the "partygate" scandal.

After claims of widespread rule-breaking surfaced late last year, Johnson consistently denied in the House of Commons that he or his Downing Street staff had violated Covid-19 lockdown restrictions.

However, he became the first UK leader to be penalized for breaking the law last week, when police disclosed that they had issued dozens of penalty notices to his employees as part of an ongoing investigation.

Johnson's single fine is related to an office celebration for his birthday in June 2020, when Britain was on lockdown due to a pandemic.

Penalties for additional events, though, could follow, and opposition parties are now calling on parliament's cross-party "privileges committee" to examine Johnson.

If lawmakers are found guilty of offenses, it has the authority to discipline them, including suspending them from the Commons.

However, the committee can only begin an investigation if a majority of MPs vote in favor of a referral.

Read more: Johnson uses Ukraine to divert attention amid his misconduct apology

The main opposition Labour Party has asked Conservatives to support its proposal for a committee to investigate whether Johnson's denials constituted "contempt of the House."

It would require a significant defection among Johnson's ruling Conservatives' 359 lawmakers to pass, which is considered unlikely.

But, with local elections coming up next month, Labour is pushing ahead with a bid to name and humiliate Tory MPs who support Johnson, intending to force many of them to abstain.

Should Johnson resign? 

Labour leader Keir Starmer described to a somber Commons on Tuesday the experience of one voter who was unable to clasp his dying wife's hand in the hospital due to the laws in place at the time.

He said the vote was "an important step towards restoring honesty and integrity into our politics."

"I am urging all Conservative MPs to do the right thing -- to respect the sacrifices their constituents made, and to vote in the national interest."

Johnson himself will be far away, starting a two-day visit to India.

He has apologized for the scandal but remains adamant he never knowingly misled parliament and has vowed to press on with issues including the war in Ukraine.

But one junior minister resigned last week following the police fine, while senior Tory backbencher Mark Harper told parliament on Tuesday that Johnson was "no longer... worthy" of being prime minister.

See more: Most of the British public wants Boris Johnson to resign

According to a recent nationwide poll, over two-thirds of the public viewed Johnson badly, compared to only 16% positively, with the term "lie" being the most often used response.

When media reports about Downing Street parties initially appeared, business minister Paul Scully admitted that the government "didn't handle it especially effectively at that moment, communication-wise."

"Nonetheless, the prime minister has gripped it, he has apologized, he's accepted the fine, he has accepted the finding of the police and he does want to move on," Scully told BBC television.