Sir Gavin Williamson is being investigated over abusive text messages to a colleague in which he complained about not being invited to the Queen’s funeral.
It's also alleged that Mr Williamson, 46, warned "there is a price for everything" in the expletive-ridden texts, sent on September 13 to Wendy Morton, then chief whip.
And the former Education Secretary was given a new role last month despite Jake Berry, the chairman of the Conservatives at the time, saying that he personally told Rishi Sunak about the complaint.
The Prime Minister has since said his government would have “integrity, professionalism and accountability at every level”.
It's said gaffe-prone Mr Williamson, MP for South Staffordshire, exhibited "bullying and intimidating behaviour" towards Ms Morton with the messages, in which he accused Ms Morton, 54, of exploiting the Queen’s death for political gain.
In the texts, which have been handed to the party, Mr Williamson said it was “very poor and sends a very clear message” that members of the privy council — the 734 politicians and civil servants who advise the sovereign — who were not “favoured” by Liz Truss, the prime minister at the time, were being “excluded from the funeral”.
Mr Williamson, who was awarded a knighthood and a place on the council by Boris Johnson earlier this year, was apparently referring to himself.
The exchange of messages, obtained by The Sunday Times, concluded with him saying: "Well let's see how many more times you f*** us all over. There is a price for everything."
Ms Morton, who resigned as chef whip as Liz Truss quit as Prime Minister, told Mr Williamson his allegations were unfounded.
The government had a limited number of tickets for the Queen’s funeral and most were reserved for the cabinet. A handful of senior parliamentarians, such as the father of the House, Sir Peter Bottomley, were also on the invitation list for Westminster Abbey in September.
Ms Morton cited the messages in an email to the complaints team at the Conservative Party on October 24, the day before Sunak was elected Tory leader and prime minister.
Mr Williamson was sacked as defence secretary by Theresa May for allegedly leaking secure information in 2019.
The politician, originally from Scarborough, North Yorkshire, appeared to apologise for the text messages last night, saying: “I of course regret getting frustrated about the way colleagues and I felt we were being treated. I am happy to speak with Wendy and I hope to work positively with her in the future as I have in the past.”
But Ms Morton, the MP for Aldridge-Brownhills, told officials last month: "I do not make this complaint lightly and wish for my name to be kept anonymous, however his messaging to me falls far below the standard of what should be expected of anyone in public office."
It is believed officials are now conducting a preliminary analysis on the evidence, further to which they could launch a formal inquiry.
According to the Sunday Times, Mr Berry, the outgoing Tory party chairman, said: "On Monday, 24th October, I was made aware, by the chief executive of the Conservative Party, [that] a complaint had been made to the Conservative Party against Gavin Williamson regarding allegations of bullying and intimidation of parliamentary colleagues.
“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."
When Mr Williamson, who worked on Boris Johnson's campaign during the 2019 Conservative Party leadership election, was knighted in March, rival parties reacted in furore.
The Liberal Democrats called the knighthood "an insult to every child, parent and teacher who struggled through COVID against the odds".
