Cabinet minister Nadhim Zahawi was repeatedly unable to say who organised a secret meeting between Boris Johnson and Partygate report author Sue Gray.
In a disastrous interview round, the Education Secretary could not provide answers on who was behind talks between Mr Johnson and the senior civil servant.
He later tried to claim the BBC was "throwing mud at something that doesn't exist" by asking for clarity on who arranged the meeting.
The Prime Minister is under pressure to explain the meeting with Ms Gray ahead of her highly anticipated report into lockdown-flouting gatherings in No10 and Whitehall.
The top mandarin is expected to publish her findings this week following the conclusion of the Metropolitan Police's investigation.
It is understood the pair met at least once to give an update on the document's progress while it was being drafted, but a Whitehall source said that its contents were not discussed at any point.
Downing Street insisted the talks were not initiated by the PM but Ms Gray's team is said to have insisted that the idea originated from No10.
Mr Zahawi was grilled on the confusion during an interview on Sky News' Ridge on Sunday.
He said: "I don't know the details of all the meetings that happen but what I do know is that the PM has never intervened in the investigation that Sue Gray conducted.
"He's always wanted her to go wherever the evidence takes her."
He said Sue Gray had the "highest level of professionalism and her is unquestionable".
Mr Zahawi added: "She didn't pull her punches in her first report you remember and then the PM came to Parliament, made the changes in fact to No10 as the report was critical of the way No10 was operating."
Pressed again, he said: "All I can say to you is the meeting that took place between Sue Gray and the Prime Minister - I can't tell you who called the meeting."
Mr Zahawi added: "I don't know who called the meeting."
Asked what was discussed, he also said: "I don't know."
Asked if he could not say who called the meeting because he did not ask No 10, or because it did not tell him, Mr Zahawi said: "(I'll) tell you what the answer is, the answer is very simple.
"The answer is the Prime Minister will never intervene in Sue Gray's investigation. The Prime Minister wants Sue Gray to basically go wherever the evidence takes her."
He later told the BBC: "I don't need to [ask No10 for clarity] because I don't believe that having a meeting with your senior civil servant is material to the outcome.
"That civil servant is independent in their investigation and has the highest level of professionalism and integrity.
"The Prime Minister has made it very clear that he has never intervened or will seek to intervene or interfere with the investigation."
Pressed again, he accused interviewer Jo Coburn of "throwing mud at something which doesn't exist".
A No 10 source insisted the request for the meeting did not come from Mr Johnson.
"The PM did not request the meeting and hasn't tried to influence the outcome in any way," they said.
"It's rightly for Sue to decide and it's all done independently."
Ms Gray was tasked with investigating the Partygate saga but her report was put on ice in January after Scotland Yard announced it would investigate.
The conclusion of the Met Police's probe on Thursday paved the way for publication of her full report, which is expected to be highly critical.
