The danger signs were there in April when Sir Keir Starmer, one of the sharpest legal brains in the Commons, tabled his motion asking the privileges committee to investigate whether Mr Johnson had committed a “contempt of the House”.
It contained no mention of the word “deliberate”, and Number 10 should perhaps have sensed that a trap was being set. But the Prime Minister was on an official visit to India, meeting his opposite number Narendra Modi, and the Labour motion went through unopposed.
When the committee published an update to the Commons in July, it produced legal advice noting that the words “knowingly” and “deliberately” were “not in the motion”, and therefore it did not have to prove either.
The legal advice stated that contempt was a “strict liability” issue, meaning in legal terms that there was no need to prove intent, or what lawyers call mens rea, a guilty mind.
Supporters of Mr Johnson, led by Sir Bill Cash, a former shadow attorney general, cried foul, pointing out that Erskine May, the “Bible” of parliamentary procedure, states: “The Commons may treat the making of a deliberately misleading statement as contempt.” It is a convention that dates back to the Profumo affair of 1963.
Erskine May says: “In 2006, the committee on standards and privileges concluded that a minister who had inadvertently given a factually inaccurate answer in oral evidence to a select committee had not committed a contempt.” The minister was required to apologise and correct the record.
According to Sir Bill, the privileges committee (now separate from the standards committee) has also ignored a resolution made by the Commons in 1997 that “ministers who knowingly mislead Parliament” will be expected to resign.
Mr Johnson’s supporters argue that ministers inadvertently mislead the Commons all the time, usually because they have been given incorrect information, and the accepted convention is for them to apologise and correct the record, not to be subjected to a full-scale inquiry.
They also believe the privileges committee is deliberately ignoring Erskine May – and therefore casting aside years of precedent – so it can go after the Prime Minister.
But Parliament sets its own rules, and the committee has been told by three eminent authorities that it can go ahead.
Sir Ernest Ryder, a former Appeal Court judge who acts as the committee’s external legal adviser, determined that contempt was a strict liability issue.
Eve Samson, the Clerk of the Journals, an internal legal adviser to select committees, and Saira Salimi, the Speaker’s Counsel, agreed.
Ms Samson argued: “It is not necessary that intention is involved,” and cited another section of Erskine May that defines contempt as “any act or omission which obstructs or impedes either House of Parliament in the performance of its functions”.
She also noted that previous committees have considered contempt cases where no intent was implied, though she also said that “intent has been considered relevant when a committee has been considering whether there should be penalties for a contempt”.
This seemingly throwaway line in a 35-page progress report by the committee may or may not prove to be the lifeline that saves Mr Johnson from having to fight a by-election to remain as an MP.