His team will produce WhatsApp messages and other “internal comms” from the time which show that the then prime minister’s statement was based on what he had been told by No 10 aides.
“When ministers go into the Commons, they are basically just reading out what they have been briefed to say,” said the source. “They trust the advice they are given, and that is justified. They have to be able to do that in order to make Parliament work effectively.
“That is what any minister would have done. Ministers need to be able to rely on what they have been told when they are updating Parliament.”
Johnson’s six-person strong legal team, headed up by Lord Pannick KC, has been given a deadline of Monday morning to submit its defence in writing, which the committee will then publish ahead of the hearing on Wednesday.
But MPs on the panel have argued that he should have known some of the events he attended broke lockdown guidance and are expected to press that point at the hearing.
The Metropolitan Police eventually issued 126 fines for Covid law breaches to 83 people linked to eight different gatherings in government buildings - proof of some wrongdoing.
Johnson will also argue that evidence submitted to the committee by Downing St staffers actually supports his case, as it shows they also believed that no rule-breaking had taken place.
“The evidence will show that the general assumption of everyone present, including those hostile to Boris, was that rules and guidance were followed,” said one of his allies said.
“That destroys the argument that Boris must have known it was not in the rules.”
His legal team will also warn that his case risks having a chilling effect on Parliament, as it would set a new precedent to haul MPs in front of the privileges committee every time they said something in good faith and then later corrected the record.
And his defence will claim that the privileges committee is an “unfair process” to subject Johnson on account of its political nature.
“We will have to make the point that Harriet Harman is already on record saying she believes Boris is guilty of contempt,” said one Johnson ally. “You would never accept a judge who passes a verdict on someone’s guilt or innocence before seeing the evidence. This is a very unfair process when you look at it in the round.”
Johnson is also likely to argue that the gatherings were held to try to boost morale in Downing St, which had been hit by a wave of illness.
The fact that many of the pictures were taken by No 10′s official photographer and published on the official Flickr account “shows that we didn’t think we had anything to hide”, said a source.
The committee is made up of seven MPs – four Tories, two Labour and one SNP – and is headed by Labour’s Ms Harman, who has been an MP for 41 years.
Johnson’s allies are urging the committee to publish all the evidence, as they believe this will convince both MPs and the public to get behind him.
The committee published an interim report earlier this month with some fresh evidence, but friends of the former Prime Minister believe this was “cherry-picked” and does not show the whole picture.
The four Conservative members of the privileges committee have come under pressure from diehard supporters of Johnson.
The Conservative Post - a website for grassroots Tories - has launched an online campaign allowing activists to send the four MPs an automated email calling on them to “protect your integrity by resigning from this committee immediately.”
Sir Graham Brady, the chairman of the 1922 Committee of Tory backbenchers, has also been lobbied to intervene and “encourage” MPs to call off the privileges committee’s inquiry.
If the committee ultimately determines that Johnson has misled MPs, they can produce a report recommending a certain number of days of suspension from the Commons.
It is up to the Commons as a whole whether to adopt or reject any recommendations. Downing St has confirmed that Tory MPs will receive a free vote to determine Johnson’s fate.
If a suspension of more than 10 days is voted through, Johnson’s constituents can trigger a byelection, provided that they gather enough signatures - imperilling his political career.
A spokesman for Johnson said: “The privileges committee will vindicate Boris Johnson’s position. The evidence will show that Boris Johnson did not knowingly mislead Parliament.”