The inquiry into Boris Johnson's partygate claims has taken a "fundamentally flawed" approach which is "wrong in principle", the legal advice to the Prime Minister says.
The advice, which was commissioned by the Cabinet Office, says that MPs on the privileges committee are "proposing to adopt an unfair procedure".
They are examining whether Mr Johnson misled the House of Commons when he insisted that "all guidance was followed in Number 10" and that there was "no party" in Downing Street during lockdown.
Written by Lord Pannick QC, one of the country's most eminent lawyers, the advice states that the proceedings would be declared "unlawful" if they were subject to a judicial review.
However, since the proceedings are protected by parliamentary privilege, they are immune from legal challenges.
The advice says: "In our opinion, the committee is proposing to adopt an approach to the substantive issues which is wrong in principle in important respects, and the committee is also proposing to adopt an unfair procedure."
It goes on to say that there are six key areas where the privileges committee "is proposing to adopt a fundamentally flawed approach".
The advice explains 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".
It had initially been assumed that MPs on the privileges committee would have to prove that Mr Johnson had "deliberately misled" the House of Commons over the extent of parties in Downing Street during the lockdowns.
This is based on the language from the "bible" of parliamentary procedure Erskine May, which states that "the making of a deliberately misleading statement [is seen] as a contempt".
But when the committee published its motion in June, it only referred to whether Mr Johnson had "misled the House", lowering the burden of proof.
The legal advice also points out that the committee has "failed to recognise that for an allegation of contempt to be established, it would need to be persuaded that the allegation is made out to a high degree of probability, that is that it is significantly more likely to be true than not to be true".
Allowing witnesses to present their evidence anonymously is an "unfair procedure", since their evidence may be relied on to establish a contempt of the House, according to the advice.
It says that Mr Johnson should be told the details of the case against him, including both the charges and the evidence, so he can have a proper opportunity to respond.
Lord Pannick writes that in order for the procedure to be considered fair, Mr Johnson should have the right to have legal representation and should, through his counsel, be able to cross-examine witnesses whose evidence is relied on to establish a contempt of the House.
The legal advice concludes: "For the reasons set out above, 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."
The legal advice is set to be seized on by allies of the Prime Minister who have already complained that the inquiry risks becoming a "kangaroo court" and alleged that it will rely merely on "hearsay evidence".
It is unusual for the Government to commission legal advice into the workings of the House of Commons and raises questions about the supremacy of Parliament.
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