Whitehall enforcer Sue Gray WILL interview Dominic Cummings over his explosive party claims, the Mirror understands.
The senior civil servant is said to believe that for her inquiry to be seen as credible she has to speak to all key witnesses.
While it is not yet clear whether she has already been in touch with the PM's former closest aide, she plans to be before publishing her findings into Downing Street gatherings.
One source said: "Sue wants the inquiry to be as robust as possible. Cummings is going to be interviewed by the time the inquiry concludes.
"It would be extraordinary if we went through the whole thing and no attempt was made to interview him."
In explosive claims, Mr Cummings had said that the PM had "waved aside" concerns about the rule-busting gathering which then went ahead.
He claimed that he and "other eyewitnesses" would be willing to swear under oath that senior officials had warned the event was against the rules.
Mr Johnson sparked widespread disbelief last week at PMQs when he apologised for the gathering but claimed he "implicitly believed" it was work related.
The under-fire PM broke cover for the first time in days on Tuesday to insist that he hadn't lied to Parliament.
In his clip to broadcasters, Mr Johnson confirmed that he had also been interviewed by Ms Gray.
"I do humbly apologise to people for misjudgments that were made," he said.
"But that is the very, very best of my recollection about this event. That's what I've said to the inquiry."
Mr Johnson claimed that "nobody told me" that the boozy lockdown bash in the No 10 garden broke Covid rules.
Downing Street has fiercely denied the claims, backed up by several senior journalists, that two officials told the PM the event was “a party” before it happened.
Deputy PM Dominic Raab said on Tuesday that Mr Johnson would "normally" be expected to resign if he had intentionally misled Parliament.
But he dismissed the claim by Mr Cummings, who has become the PM's fiercest critic, that the PM had lied to the Commons as "nonsense".
However, Mr Raab described the event as "a party" before rapidly backtracking.
Many Tory MPs are believed to be waiting for the result of Ms Gray's inquiry, which could be pushed back until next week, before deciding whether to trigger a confidence vote in the PM.
Exactly 54 letters need to be submitted, with reports suggesting that around half that number are already in.
