Conservative party chair Grant Shapps instructed Hill Dickinson to send letters threatening to sue a constituent over comments made about him in a Facebook post, it has emerged.

Dean Archer, a chauffeur and former Labour councillor residing in Shapps' Welwyn Hatfield constituency, reportedly wrote a post on the Facebook group 'Positively Hatfield for Hatfield' questioning Shapps' honesty in running an online marketing company under the name Michael Green.

According to the Guardian, letters to Archer from Hill Dickinson on Shapps' behalf called the comment a "defamatory allegation" and demanded a retraction and an apology from Archer.

In the letters, dated October and November, the firm asked for "proposals for compensating our client in lieu of damages and (an) undertaking to indemnify our client in full for his legal costs".

The letters said the damage cause by the "libellous" comment had not ended. They quoted comments by former Lord Justice Bingham, who said: "Defamatory statements are objectionable not least because of their propensity to percolate through underground channels and contaminate hidden springs."

Speaking to the Guardian, Archer said: "At first I laughed it off but then my wife got worried. He was threatening us. We stood to lose our house."

A spokesperson for Hill Dickinson said the firm was "unable to provide comment" due to "client confidentiality".