Labour deputy leader Tom Watson said it was not his role to judge whether convicted fantasist and fraudster Carl Beech was telling the truth.
The MP said he only once met the man known as “Nick”, in 2014, two years after speaking out in Parliament about the existence of a high-profile paedophile ring.
Mr Watson also declined to apologise to former Tory MP Harvey Proctor, one of Beech’s victims, but appreciated he was “hurt and angry”.
Mr Watson said in a statement: “I met the man I knew as ‘Nick’ once, on 8 July 2014, two years after I had raised my question in Parliament.
“During that meeting Nick said very little and did not name any of his alleged abusers.
“I reassured Nick that the police had made clear that all allegations of historic sex abuse would be taken seriously and treated sensitively.
“That is what the police had asked me to do, and it was the process I followed with all those who claimed to be survivors of historic child sex abuse.
“It was not my role to judge whether victims’ stories were true.
“I encouraged every person that came to me to take their story to the police and that is what I did with Nick.
“I hope this trial, and the case of one person, does not prevent survivors of child sexual abuse coming forward and reporting their experiences to the police.”
In an emotional statement, Mr Proctor called on the Labour deputy leader to apologise and said Mr Watson had said in a Guardian interview that Beech “was not delusional”.
Mr Watson said: “Harvey Proctor makes an understandably emotional attack today.
“I appreciate that he is hurt and angry but in justifying his attack he has disingenuously used a selective quotation from an interview I did…
“What I actually said in that interview was: ‘What I’m certain of is that he’s not delusional. He is either telling the truth, or he’s made up a meticulous and elaborate story. It’s not for me to judge.’
“My actual quotation made clear that I did not know if ‘Nick’ was telling the truth, or lying, but that it was for the police, rather than me, to make a judgment about that.”
Mr Watson said “Nick” never alleged to him that Mr Proctor had abused him.
He said he was justified in speaking out in Parliament about a paedophile ring, as his pressure had led to lost police files being recovered, and three men being convicted for child sexual abuse.
He said: “As a public figure of responsibility I felt it was my duty to respond to the claims that had been made to me.
“To stand aside when there was a possibility that children were at risk of sexual abuse was not an option.”