A probe into whether Boris Johnson lied to parliament over the partygate scandal would be found “unlawful” by a court, a top barrister has said.

In a published legal opinion commissioned by the government, Lord Pannick – a crossbench peer who sits in the House of Lords – described the Privileges Committee’s approach to its investigation into whether the PM misled MPs as “unfair” and “flawed”.

Lord Pannick’s advice states: “We advise Mr Johnson that the committee is proposing to proceed by reference to substantive errors as to the ingredients of contempt and the standard of proof required, and is proposing to adopt an unfair procedure.

“But for parliamentary privilege, a court hearing a judicial review application brought by Mr Johnson would declare the committee’s report to be unlawful.”

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His advice says that “the committee has failed to understand that to prove contempt against Mr Johnson, it is necessary to establish that he intended to mislead the House”.

The top barrister also warned that “the threat of contempt proceedings for unintentional mistakes would have a seriously chilling effect on all members”.

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The publishing of legal advice commissioned by the government is a highly unusual move.

Labour MP Chris Bryant, who chairs the privileges committee but is not leading the partygate inquiry having recused himself, dismissed the government-commissioned legal opinion by Lord Pannick as “disgraceful bullying” and “wrong on several counts”.

“Lord Pannick’s bizarre ‘opinion’ has no formal status and is wrong on several counts,” he posted on social media.

“Firstly, he fails to mention that the motion that charged the committee makes no mention of ‘intentionally misleading’.

“Nor does he acknowledge that many aspects of standards processes have changed over the years, including the introduction of the right of ministers to correct the record through a written ministerial statement – which was used 200 times last year.”

He continued: “It’s time this disgraceful bullying stopped. Let’s hear and see the evidence. If Johnson has a good case to make, he’ll be vindicated. If not, he should take his punishment.”

Meanwhile, shadow leader of the House of Commons Thangam Debbonaire told BBC Radio 4’s World at One programme that not allowing the Commons inquiry to investigate whether Mr Johnson corrected the record over his partygate denials would amount to a cover-up.

Lord Pannick is a crossbench peer who has previously acted against the government for anti-Brexit campaigner Gina Miller and Shamima Begum over the removal of her British citizenship.

Although Mr Johnson is due to leave Number 10 next week, the Privileges Committee has said it will go ahead with its inquiry into whether Mr Johnson committed a contempt of parliament by telling MPs on several occasions that there were no lockdown-busting parties in Downing Street and across Whitehall.

If the committee finds there has been a contempt, it can recommend a sanction on the PM – but it is up to the House of Commons to accept or reject that recommendation.

Such a sanction could include Mr Johnson being suspended from the Commons or even kicked out in a by-election after a recall petition.

Political correspondent

This opinion paper is a damning verdict on the way in which the privileges committee will conduct their inquiry into Boris Johnson.

But, as one senior MP put it today, it’s just “noise” – and won’t have any material impact on the investigation.

As the document states, committee inquiries like these can’t be subject to legal challenge because they are matters of parliament.

But the fact this opinion paper has been sought and published by the government at all has raised eyebrows.

One ex-Cabinet Office official said Boris Johnson really should have commissioned and funded this personally.

In the long run, Westminster watchers will wonder if this is more evidence of the outgoing PM trying to safeguard his reputation with a view to one day returning to frontline politics.

The publication of the advice has fuelled questions over whether the final act of the departing PM will be to try and attempt to influence the proceedings of the investigation against him.

But supporters of Mr Johnson have argued that Lord Pannick’s legal advice means the investigation, which will be chaired by senior Labour MP Harriet Harman, should end.

Speaking before the advice was published, Culture Secretary Nadine Dorries told the Daily Mail: “This expert legal opinion shows that the inquiry was a biased, Kafkaesque witch-hunt – it should now be halted before it does any more damage.”

The COVID law-breaking parties in Downing Street were among the scandals that forced Mr Johnson’s resignation as Tory leader back in July.

His successor, either Rishi Sunak or Liz Truss, will be announced on Monday 5 September – with voting closing at 5pm on Friday.

Tory leadership frontrunner Liz Truss signalled during party hustings she would like to scrap the inquiry.

The committee carrying out the probe is made up of seven MPs – four Tories, two Labour and one from the SNP – and has called for evidence of “Mr Johnson’s knowledge of the activities in 10 Downing Street and the Cabinet Office under COVID-19 regulations, from the occurrence of those events until now”, as well as “any briefing given to, or inquiries made by, Mr Johnson relating to those events”.

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