MPs will vote on Thursday on whether to refer Boris Johnson for a formal parliamentary investigation into whether or not he misled them about his knowledge of Covid law-breaking parties. Lindsay Hoyle, the House of Commons Speaker, granted a request by Labour for the matter to be examined given the prime minister insisted repeatedly that no rules were broken in Downing Street during lockdown, but was later issued with a fine by police for attending a birthday party. The ministerial code, which bears Johnson’s signature, says the punishment for misleading parliament is resignation. But since the prime minister is the only person who can enforce the code and he has resisted calls to stand down, the fresh investigation would pile further pressure on him. Labour and the Liberal Democrats had requested for Hoyle to grant a debate and vote on referring the row to the privileges committee, a cross-party group of seven MPs with a Conservative majority but chaired by the Labour MP Chris Bryant. Hoyle said: “It is not for me to determine whether or not the prime minister has committed a contempt. My role is to decide if there is an arguable case to be examined. “Having considered the issue, having taken advice from the clerks of the house, I’ve decided that this is a matter that I should allow the precedence accorded to the issue of privilege.” The Labour leader, Keir Starmer, will table a motion on Wednesday to be discussed and voted on the following day, which will call for the privileges committee to look into whether the prime minister misled parliament. Johnson is meant to be on a trip to India, so will miss the vote and avoid having to defend himself in the chamber. Downing Street and the Conservative whips’ office may heavily whip their MPs to oppose the move, telling them it will be treated like a confidence vote in Johnson and that any rebels will be thrown out of the parliamentary party. But being forced to show their hand will place those Conservative backbenchers who have so far remained silent about their position on Johnson’s law-breaking in a difficult position. Opposition parties have already begun crafting digital adverts targeting constituents whose Tory MPs will vote against the motion, to hammer them in the run-up to next month’s local elections. Tory MPs were urged by the Liberal Democrat leader, Ed Davey, to “do their patriotic duty” and kick Johnson out of Downing Street. He said: “The British public have declared Boris Johnson a liar. Now it’s time for parliament to do the same. The country cannot afford a prime minister who breaks the law and lies about it, especially when families are facing a cost of living crisis. “Johnson has taken the British people for fools for far too long, and it’s time for Conservative MPs to show where they stand.” In the event that MPs do vote for an investigation, the privileges committee will be able to call for papers, summon witnesses and recommend a sanction – such as suspension or expulsion.