Two of the six MPs who are set to investigate Boris Johnson over partygate tried to get disgraced former minister Owen Paterson off the hook last year.

They sit on the Privileges Committee, which will launch an inquiry into whether the prime minister misled Parliament about lockdown-busting parties in Downing Street.

Laura Farris and Sir Bernard Jenkin, both Conservative MPs, voted in November to overrule the Standards Committee in a botched attempt to save Paterson from suspension – despite his rule-breaking lobbying work.

The controversial vote also aimed to rewrite Parliament’s rulebook in a move that was widely condemned by opposition MPs and even some rebel Conservative MPs. Labour’s deputy leader Angela Rayner described it as an “absolute disgrace”.

Farris and Jenkin’s previous votes to try to save their colleague raise questions over whether the inquiry into Johnson’s conduct will be impartial.

SNP backbencher Martin Docherty-Hughes told openDemocracy: “All members of the Privileges Committee should reacquaint themselves with the seven Nolan Principles [the ethical standards expected of public office holders] and make sure that they are fully able and capable of implementing them impartially.” The principles include integrity, objectivity and accountability.

Allegations against Paterson were first investigated by the Standards Committee, before all MPs were asked to vote on what sanctions he should face. The committee, whose membership overlaps with the Privileges Committee, had recommended a 30-day suspension.

In such cases, members of the committee would normally be expected to abstain from voting.

But Jenkin was able to support Paterson because he had recused himself from the committee’s inquiry on the basis that he was Paterson’s close friend.

He was later accused of being a “stooge” for Boris Johnson, with claims that he had “betrayed his duty” to protect the role of parliamentary committees.

Farris only joined the committee last month, meaning she was free to vote on Paterson’s suspension in November. At the time, she told her local newspaper she had backed him because the Conservative Party had whipped her to do so.

Bizarrely, given her subsequent appointment to the Standards Committee and Privileges Committee, she added: “I feel very uncomfortable being asked to vote on colleagues… It’s still the House norm for MPs to pronounce on their colleagues, and I find that very odd. Imagine in your workplace if someone was disciplined and you had to decide the sanction. It’s quite weird.”

Both will now be responsible for deciding whether Johnson broke parliamentary rules, like Paterson did. Johnson, who was handed a fine by the Metropolitan Police earlier this month for having a birthday party indoors during the first Covid lockdown, is alleged to have misled the Commons when he repeatedly told MPs that no rules were broken.

The committee’s chair, Chris Bryant, has already recused himself from the Partygate inquiry because he has made several public statements about the scandal. The Labour MP said it was important that the committee should be able to proceed without any “imputation of unfairness”.

It means only six MPs will be left on the committee to conduct the investigation. Four of them are Conservatives, including Farris and Jenkin, with just one member each from Labour and the SNP.