The former MP Chris Mullin has been called “a disgrace” by the sister of a victim of the Birmingham pub bombings after telling an inquest that he would not reveal the names of the surviving attackers.

Twenty-one people were killed and about 220 injured when bombs were detonated by the IRA in two city centre pubs – the Mulberry Bush and the Tavern in the Town – on the evening of 21 November 1974.

In 1975, six men – who became known as the Birmingham Six – were convicted for the bombings, but were acquitted 16 years later. Mullin’s work as an investigative journalist and his book, Error of Judgement, contributed to their release.

Speaking at an inquest into the deaths of the 21 on Wednesday, Mullin said he interviewed 16 or 17 IRA members, including those who carried out the attacks, while researching his book, in order to establish the innocence of the Birmingham Six.

“I know who carried out the bombings, yes, and I think that is in any case becoming public knowledge. I suspect the police know about it now,” said Mullin, who was the Labour MP for Sunderland South from 1987 to 2010.

Asked if he had promised those men that he would not name them until they had died, he said: “I would say that I volunteered an understanding that I was not interested in naming their names, but what I wanted was to hear from the people who I believed did actually carry out the bombings.”

Leslie Thomas QC accused Mullin of protecting the identity of those responsible for murder, at which point Julie Hambleton, the sister of the 18-year-old victim Maxine Hambleton, shouted, “You are a disgrace!” from the public gallery, before leaving the courtroom.

Mullin told the inquest he understood why the families of the victims wished to know who had carried out the attacks. “If I was one of them, I would too. I wish I could get to the rooftops and shout out,” he said.

He said not naming the bombers had been “the basis of their cooperation” with his investigation. “I would not have been able to get to the bottom of this had I not given those assurances,” he said.

Mullin was asked about his interview with a man labelled “the young planter”, who helped to plant the bombs and was a teenager at the time. He said he told him he had been assured that the pubs would be evacuated before the bombs went off.

“He was in a bit of a state,” said Mullin. “It was clearly something that has had a big impact on him for the remainder of his life, understandably, perhaps.” Thomas responded: “Well, killing 21 people and seriously injuring 200, one would hope would have some impact.”

Giving evidence to the inquest last week, a convicted IRA bomber known as Witness O named four men he said were responsible for the bombings as Seamus McLoughlin, Mick Murray, James Gavin and Michael Hayes. McLoughlin, Murray and Gavin have since died.

The inquest into the deaths was opened in November 1974 but was adjourned to allow for a criminal investigation. Fresh inquests into the deaths were ordered in 2016 but were delayed by disputes over whether the hearings should examine who might be responsible for the bombings. In September, the court of appeal ruled that alleged perpetrators would not fall within the framework of the inquest.

The inquest continues.