A Parliamentary committee’s crunch probe into whether Boris Johnson misled MPs over Partygate would be ruled unlawful by judges, a top QC warned today.
Lord Pannick claimed a court would slam the Commons Privileges Committee's approach to its inquiry.
He said the investigation was "unfair" and "fundamentally flawed".
MPs are probing whether the outgoing Prime Minister committed contempt of Parliament by lying to the House of Commons over whether lockdown-busting parties took place in Downing Street.
They have been told they can find Mr Johnson in contempt even if he is deemed to have accidentally misled the House.
But critics warned that inadvertently misleading MPs should not be considered contempt because it could hamper ministers’ statements at the Despatch Box.
In a highly unusual move, Downing Street ordered an expert legal opinion on the issue. Even more unusually, today it published the advice.
In a minor victory for the doomed Premier, who is booted out of No10 on Tuesday, top barrister Lord Pannick said: "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."
He said MPs need to establish "that Mr Johnson intended to mislead the House - that is that he knew that what he told the House was incorrect".
Warning of the potential impact on other MPs if the committee pursued its approach, the crossbench peer added: “The threat of contempt proceedings for unintentional mistakes would have a seriously chilling effect on all Members."
However, the committee’s approach to its inquiry cannot be referred to a court because Parliament is sovereign.
Labour MP Chris Bryant claimed the Government-commissioned legal opinion was "wrong on several counts".
The Privileges Committee chairman, who has recused himself from the Partygate probe, tweeted: "Lord Pannick's bizarre 'opinion' has no formal status and is wrong on several counts.
"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."
Mr Bryant accused the barrister of wanting to "change the ancient principle that a Member of either House defends their conduct before their colleagues in person, not via a lawyer".
The committee's investigation threatens to further tarnish the PM’s legacy and could derail his future as the MP for Uxbridge and South Ruislip, West London.
If he is found to have lied to Parliament, Mr Johnson could be suspended from the Commons or even kicked out in a by-election after a recall petition.
Downing Street has repeatedly signalled it will assist the parliamentary inquiry, which ultimately stems from the Mirror’s revelations of lockdown-busting, boozy bashes in No10.
Mr Johnson and his wife Carrie were slapped with fines for breaking the rules.
