IT'S a plot worthy of Armando Iannucci: two lobbyists fall out, one gets elected as an MP, launches a Bill to curb the malign influence of his former trade, and uses his platform at Westminster to traduce the reputation of his erstwhile colleague.

But it doesn't come from a Thick of It script – it's from Hansard last Friday. The attack by the MP for Dunfermline and West Fife, Thomas Docherty, on his former colleague Mark Cummings has now resulted in a much wider spat involving the Speaker of the House of Commons, Labour leader Ed Miliband and the party's Scottish leader Johann Lamont.

Mr Cummings has demanded action on all these fronts for what he sees as an abuse of parliamentary privilege. He has challenged Mr Docherty to repeat the allegations outside parliament and be prepared to defend the claims in court.

When Mr Docherty launched into his initial attack some MPs were clearly uneasy, Conservative James Duddridge pointedly asking: "Is the Honourable Gentleman using his privilege here to say something in the House that he is not able to say outside, or are these comments that he would be equally happy to share outside?"

But Mr Docherty pressed on, speaking generally about council bribes and then accusing Mr Cummings of "playing off his contacts" and using others to call politicians or impersonate journalists. He added: "Mr Cummings also seems to revel in bullying. He likes to intimidate people who disagree with his clients' views."

Both men have worked for lobbyist PPS. The MP repeated the story about an "incredibly offensive" text message he once claimed to have received in relation to the "Famine Song" controversy, adding: "I am not alone in receiving intimidatory behaviour from Mr Cummings, who is well known for bullying and becoming aggressive, particularly towards women opponents of schemes."

Mr Cummings said: "I am absolutely appalled at the conduct of Mr Docherty. To attack me personally and my company in this way without warning and under the cloak of parliamentary privilege is a serious abuse of his power as an MP.

"I will not hesitate to take legal action against him if he repeats these allegations outside of the Commons, and I've written to Mr Speaker and the leaders of the Labour Party in the UK and Scotland asking for disciplinary action to be taken against Mr Docherty.

"I will not allow these unfounded allegations to go unchallenged as they could pose a serious threat to my business."

Asked if he would be prepared to repeat his comments away from Parliament and the hostile view expressed by some of Mr Cummings's colleagues, Mr Docherty said: "They can imply whatever they like. Parliamentary privilege exists to allow MPs to expose hypocrisy."