Speaking on the BBC’s Sunday Politics show, Mellor, who is a regular contributor on LBC Radio, said Newsnight’s brand was tainted beyond redemption after relying on Messham as a witness.
sunday politics
David Mellor labelled Steve Messham a 'weirdo' on the Sunday Politics show
He told presenter Andrew Neil: “They relied on a man who the Mail on Sunday have revealed in two and half pages as a weirdo.
“He’s already cost one and half million in libel damages and costs when he accused a policeman of sexually abusing him.
"Why didn’t [Newsnight] show him a photograph (of Lord McAlpine) it’s extraordinary. The idea of Alistair McAlpine being involved in child abuse is so ludicrous somebody on Newsnight should have realised."
Mr Messham was called 'a victim of his own delusions' in the Mail on Sunday piece
Campaigners against child abuse and Twitter users were quick to condemn Mellor and the Mail on Sunday for attacking Messham.
Many insisted his description of Messham as a weirdo would discourage other victims of child abuse from coming forward.
Peter Watt, director of the NSPCC helplines aid in a statement to the Huffington Post UK:
"Adverse comments on victims disclosing the horrendous attacks they suffered as children are extremely unhelpful. This could stop other victims from coming forward and prevent abusers being brought to justice.
"This sends the wrong message to young victims of abuse who may be discouraged from speaking out. And it could also make it more difficult for adults to speak out on behalf of children and report their concerns.
"Being sexually abused can lead to long-term physical and emotional damage. We know that it can take years for victims to speak out about abuse.
"We urge victims of child abuse or anyone with relevant information to contact the NSPCC on 0808 800 5000."
The End Violence Against Women Campaign tweeted:
EVAW Coalition
People & newspapers who are criticising a survivor of sustained child sexual abuse need to check their morals 
Mark Williams Thomas, a former detective and child protection expert who presented ITV's expose of Jimmy Savile, tweeted:
Mark Williams-Thomas
Treatment of child sexual abuse victims is not something to be handled lightly. They need care & support & must not be used for others gain
Louise Mensch said Mellor's comments were "revolting"
Louise Mensch
 that is stunning. What did the presenter do to challenge? Who was it? Revolting on Mellor's part
Andrew Neil tweeted after the show that it was important to look at the comments in context.
Andrew Neil
All working up a lather over D Mellor's wrong dismissal of Newsnight source as "weirdo" should watch last 10 minutes Sunday Politics 
Later on in the programme, he said in a discussion with journalists:
"Mr Messham was the witness on which Newsnight relied and it turned out he was an unreliable witness. I don't agree with David Mellor who called [Steve Messham] a weirdo this morning.
"He's been shown to be an unreliable witness several times. He's also suffered the most terrible child abuse which I think would make us unreliable sometimes in any circumstances. I think there's a difference between being unreliable given what you've gone through and being described as a weirdo, which I don't agree with."
The Mail on Sunday's piece on Messham, referenced by Mellor, also attracted criticism after calling him a "victim of his own delusions".
Dave Jones
Horrible smear campaign and hatchet job by the Mail of Sunday upon Steve Messham. Could well prejudice further investigations sadly.
Edwina Currie, who has written for the Daily Mail herself, labelled the article a "disgrace".
David Mellor began trending on Twitter with some quick to highlight the MP's more eccentric indiscretions.
In one of the most explosive political sex scandals, The Sun reported in 1992 that Mellor liked to have sex with actress Antonia de Sancha while wearing a Chelsea Football strip. It was also reported that de Sancha had sucked Mellor's toes.
Isn't it a bit rich for someone like David Mellor to pass comment following his series of scandals in the past?
Mellor's comments came after Steve Messham apologised to peer Lord McAlpine on Friday - saying he had mistaken his identity and cleared him of any wrongdoing.
Lord McAlpine, a former Tory treasurer and deputy chairman, has been the subject of intense speculation and innuendo since Messham claimed last week he had been abused by a senior Conservative from the Thatcher era.
But the peer released a strongly-worded statement rejecting any suggestion that he had abused Mr Messham or any other residents of the Bryn Estyn home in Wrexham, North Wales.
In a further statement on Friday night, his representatives said solicitors were preparing writs with a view to taking legal action against "all media who have defamed Lord McAlpine's reputation and published defamatory statements".