An Indian-origin civil rights campaigner has threatened to sue a cabinet minister in the United Kingdom for allegedly insinuating that her relationship with former shadow Home Secretary David Davis was inappropriate.

Shami Chakrabarti, 39, the prominent director of civil rights group Liberty, has demanded a public apology from Andy Burnham, the Culture Secretary, who she accuses of tawdry character assassination and innuendo.

The director of the leading human rights group, who is married and has a son, has written a strongly-worded letter to Burnham after he accused her of having 'late-night, hand-wringing, heart-melting phone calls' with Davis.

Chakrabarti has demanded a written apology from Burnham to herself, and to Doreen Davis, wife of David Davis, who resigned his parliamentary seat this week to fight a by-election on the issue of civil liberties.

In an article in the left-wing 'Progress' magazine, Burnham allegedly implied that Davis' stance was a result of his close relationship with the civil liberties chief and their curious phone calls.

In her letter, copied to Prime Minister Gordon Brown, Home Secretary Jacqui Smith and Chief Whip Geoff Hoon, Chakrabarti says that she is surprised at Burnham's actions, given that he is himself married with young children.

She goes on to warn that she will launch legal proceedings if the allegations are repeated.

"I am writing in relation to your recent article in the ironically titled Progress magazine. In that article you set out to smear my dealings with the former Shadow Home Secretary," the letter said.

"I must say that I find this behaviour curious, coming as it does from a Cabinet Minister, let alone someone with a partner and family of his own. By your comments you debase not only a great office of state but the vital debate about fundamental rights and freedoms in this country," Shami said in the letter.

"Indeed you seem reluctant to engage in that debate except in this tawdry fashion. I look forward to your written apology, as I'm sure does Mrs Davis. If on the other hand you choose to continue down the path of innuendo and attempted character assassination, you will find that the privileged legal protection of the parliament chamber does not extend to slurs made in the wider public domain," the letter said.

The comments caused dismay among Labour backbenchers.

"I am extremely disappointed in Andy Burnham. I thought the Labour Party had left these sort of politics behind. He wouldn't talk like that if the head of Liberty was a man," Diane Abbott, a Labour MP, said.

A spokeswoman for Burnham said the minister regretted if any offence had been caused.

"Andy Burnham was making a political point about David Davis' inconsistent views on capital punishment and civil liberties. An interpretation has been placed on Andy's remarks that he did not intend. His comments related to politics and nothing else. He regrets if any personal offence has been caused," she said.