A judge has apologised to the family of a murdered man after they overheard him say that victim impact statements made no difference when considering parole for his killers.

Judge Graham White's conversation was overheard by Geraldine and Peter McGinty, whose son Colin was stabbed to death 13 years ago in Merseyside.

The 21-year-old's parents told the BBC they were heartbroken to hear the comments made over the video link, after submitting a statement that said they were "serving a life sentence of heartache and grief and pain" as the killers applied to be transferred to open prisons.

Mrs McGinty said: "The judge turned round and said to someone else in the room: 'I feel so very sorry for these families. They make these statements thinking they are going to make a difference, but they make no difference at all. Someone should tell them. The heartache that we go through to do these statements, to be told they don't make any difference."'

Judge White told the BBC he was sorry for the effect his comments, which he said were made in a private conversation, had had on the family, but added that while the statements had an impact they did not affect the parole board's assessment of the prisoner's risk.

The chairman of the parole board, Sir David Calvert-Smith, said there would be an investigation into the judge's comments.

In guidance issued by the Ministry of Justice last year, a Victim personal statement was defined as giving victims "the opportunity to explain how the crime affected them and their family, and what the impact of release will be".

The shadow justice secretary, Sadiq Khan, said the judge was wrong when he suggested that victim impact statements made no difference.

He told BBC Radio 4's Today programme that without hearing from the victim the parole board could not find out from the offender whether he or she understood the damage done to a family.

He also said the statements could influence licence conditions if somebody was to be sent to an open prison or allowed day release.

The Labour frontbencher, whose party has called for a victims' law, added: "I think victims should have a role to play. Too often … rules are written in codes of practices, or in charters or in manuals. They are simply paid lip service to rather than professionals... understanding the cultural change that needs to take place."

He said victims needed to feel they were at the "heart of the justice system".