Lawyers are investigating whether Nadine Dorries broke the law by sending "forceful" messages to senior civil servants after she was snubbed for a peerage.
MPs heard she had "threatened" to use her TV show and her position in the House of Commons to "get to the bottom" of why she wasn't granted an honour in Boris Johnson's resignation list.
Angry Ms Dorries announced she would stand down as an MP with immediate effect after her elevation was blocked - but is yet to do so. In the aftermath she launched a scathing attack on Rishi Sunak, dismissing him as a "privileged posh boy" after her nomination was blocked.
Top civil servant Simon Case said he had taken legal advice and was seeking more guidance over whether the MP had committed an offence under the Honours (Prevention of Abuses) Act 1925.
In a savage dig at Ms Dorries, Tory MP William Wragg - who chairs Parliament's Public Administration and Constitutional Affairs Committee - described her as the "lingering MP for Mid Bedfordshire".
He asked Mr Case: "The most recent set of resignation honours - and in particular peerages - has excited some public comment and indeed some current, former and lingering members of the House of Commons as well.
"Are you aware of any rather forceful communications, it might be described, sent by the lingering member for Mid Bedfordshire to senior civil servants really threatening to use the platform of the Commons and indeed her own TV programme to get to the bottom of why she hadn't been given a peerage?"
Mr Case responded that he had been made aware of the matter, and was seeking advice.
"Yes I am aware of these communications and have flagged them both to the chief whip and the Speaker of the House."
Mr Wragg then asked if the Cabinet Secretary had asked for any legal advice, questioning whether the 1925 act "came into play".
Mr Case responded: "Seeking further advice on that question. Taken initial advice but asked for more."
The Mirror has contacted Ms Dorries and Tory chief whip Simon Hart for comment.
The act Mr Wragg referred to was passed in the wake of a "cash for honours" scandal under David Lloyd George, but has seen just one person convicted - Maundy Gregory in 1933.
The Liberal Democrats called on the Prime Minister to withdraw the Tory whip while the claims are investigated.
Deputy leader Daisy Cooper said: "These allegations are staggering and it's crucial a swift investigation takes place into whether Nadine Dorries may have broken the law.
"Not only is Dorries failing to represent the people of Mid Bedfordshire, but now it emerges she has allegedly sent threatening messages to civil servants.
"The least Rishi Sunak can do is suspend her by withdrawing the Conservative whip while any investigation takes place."
On June 9 Ms Dorries - who has been notable by her absence from the Commons since Mr Johnson was forced to quit - announced she was standing down with immediate effect, before saying later in the evening she had changed her mind.
The row was ignited after Mr Johnson put her forward for a peerage, only for Rishi Sunak to block the move. Ms Dorries fumed that evening: "A Prime Minister doesn't do that. They'd be a very foolish prime minister who tampered with the list of a former Prime Minister because he'd end up not getting his own nominations through."
She later labelled the Prime Minister and his political aide James Forsyth "privileged posh boys".
Earlier today it emerged that she will publish a book settling scores over Mr Johnson's downfall just three days before the Tories' annual conference.
The former Culture Secretary - who was named in a bombshell report for trying to undermine Parliament's Partygate probe - continues to maintain the ex-PM was wrongly removed.
She claims the "darkest political arts" run through the Tory Party. Her book, named The Plot, is described as a "story of treachery and deceit" by publisher HarperCollins. She claims to have discovered a "fault line" and a "history of deception" within the party as she takes aim at those she blames for Mr Johnson's removal.
It is likely to make uncomfortable reading for Mr Sunak, who Ms Dorries has repeatedly lambasted since her mentor's departure from No10. According to a statement released this morning, the book will tell how the "most charismatic politician of his generation" was "driven from office" and is supposedly based on the testimony of "dozens of inside sources".
Ms Dorries claims that "powerful, unaccountable forces behind the scenes" were behind Mr Johnson's downfall.
Ms Dorries said: "When I started this book I had no idea of the journey I was embarking on. But the more people I spoke to in the heart of Westminster, the deeper the story unfolded. I had wanted to discover the forces behind the downfall of the Prime Minister.
"Instead, I found a fault line within the Conservative Party stretching back decades and a history of deception fuelled by the darkest political arts.
"If you thought power flowed from the people into Parliament, be prepared to think again."
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