The prime minister did not see threatening texts sent by Gavin Williamson to another MP before appointing him to cabinet, a minister has said.
Calls are being made for an "urgent independent investigation" after a series of expletive-laden messages from Williamson to the then-chief whip Wendy Morton were leaked to the press.
Complaining that he had not been invited to the Queen's funeral, Williamson struck a threatening tone when he said: "Well let’s see how many more times you f**k us all over. There is a price for everything."
He suggested that political opponents were deliberately being excluded from attending, to which Morton repeatedly said that this was "not the case".
It calls into question Rishi Sunak's assurances that his government would be characterised by "integrity, professionalism and accountability at every level".
Jake Berry, who until recently served as Conservative party chairman, claims he told Sunak about the complaint against Williamson before he was appointed as minister without portfolio.
But Oliver Dowden, chancellor of the Duchy of Lancaster, has said the PM didn't know the specifics of the messages until they were leaked over the weekend.
Watch: Rishi Sunak was aware Gavin Williamson faced bullying claim, MP says
Dowden told the BBC's Sunday with Laura Kuenssberg show: "It was no secret that Gavin Williamson, and others, indeed didn't enjoy a good relationship with the chief whip at the time.
"It was not the case that the prime minister actually saw the detail, [or] actually saw the texts themselves."
He added: "These were sent in the heat of the moment at a very difficult time, Gavin accepts that he shouldn't have said these things, and he regrets it. We should all treat each other with respect and courtesy and that was not the case there."
The senior minister said he didn't want to "prejudge" an ongoing investigation into the text messages currently being investigated by the party.
Labour's shadow climate secretary Ed Miliband has said an independent investigation is needed, warning of a potential "cover-up" or "whitewash" if the issue is only handled internally.
He told Sky News's Sophy Ridge on Sunday programme: "We already know about the whole Suella Braverman issue: being reappointed six days after she resigned.
"It was very noticeable... that Oliver Dowden couldn't deny that Rishi Sunak knew about those issues to do with potential bullying against Wendy Morton, the chief whip, by Gavin Williamson and yet he reappointed him."
Miliband added: "What it says is that Rishi Sunak was making decisions simply in his own narrow short-term interest as far as the Conservative Party leadership was concerned, not the national interest.
"There needs to be an urgent independent investigation into exactly what happened. We can't have a cover-up, we can't have a whitewash here.
"What did Rishi Sunak know? When did he know it? What did Gavin Williamson do and what are the implications of that?"
Some opposition MPs have cast doubts on whether the prime minister really didn't know anything about the contents of Williamson's text messages.
Labour deputy leader Angela Rayner tweeted: "Rishi Sunak claims not to recall a conversation just days ago with the Tory Chair about a serious complaint against his own Cabinet appointee. That beggars belief.
"But it’s not just the PM’s memory in question. His appalling judgement in reappointing Gavin Williamson is too."
Jess Phillips, Labour MP for Birmingham Yardley, wrote: "Oliver Dowden says Gavin Williamson's behaviour is unacceptable but seemingly is accepting it by saying it's in the past.
"Williamson thinks his tone makes him tough, on the contrary anyone reading it thinks it makes him look like a child spitting out his dummy."
Some within the Tory party are also openly criticising Williamson, with former culture secretary Nadine Dorries tweeting: "Beyond stunned. As @DCMS SoS the funeral was my departmental responsibility until days before the Queen passed.
"I did not expect to attend. Entitlement is a despicable quality."
Former party chairman Berry said he was informed by the party chief executive on 24 October that a complaint had been made against Williamson regarding claims of "bullying and intimidation of parliamentary colleagues".
He told the Sunday Times: "In compliance with protocol, in my capacity as party chairman, I informed both the new prime minister and his incoming chief of staff about the complaint on the same day."