The issue reached Westminster on Wednesday - a day after it emerged that police in Northern Ireland had kept human tissue, including bones and organs, in 64 unexplained or suspicious death cases without informing relatives.

Assistant Chief Constable George Hamilton said this was allowed to happen between 1960 and 2005 due to a "systemic failure".

He offered an apology to families, saying some had been treated in a "bad way because of a lack of information".

"The context is important this was a different time, a different place, going back 50 years," ACC Hamilton said.

"None of that is excuses for the upset and anguish that has been caused to the families who are now being presented with what could be very difficult information this week."

There have been 4,000 murders in Northern Ireland since 1960. Before 2006, it was not practice to tell relatives that body parts had been kept.

State Pathologist Jack Crane said human tissue would be kept for two reasons.

There’s something that may be unique or unusual that we need to keep to assist police.

Jack Crane

"One would be because it would assist or help us determine the precise cause of death and the other would be for evidential purposes.

"Some of these cases have not come through the criminal justice system, some of these cases could still come to court," he added.

ACC Hamilton said the PSNI was satisfied that there was a good reason for the tissue retention and revealed that an audit showed the victims' identity and their next of kin.

He added that since 2006 the PSNI has been compliant with the Human Tissue Act, but that body parts retained for the purposes of police investigation are exempt from the act. He said it was "good practice" to inform families.

At Westminster on Wednesday, DUP MP Nigel Dodds called on the Prime Minister to join him in "demanding the fullest answers" about what happened and suggested an independent review should be held.

David Cameron expressed sympathy for the families on behalf of the House and said: "It must be a time of huge anguish for them."

He added: "I'm extremely sorry that this report has been leaked because it was going to be announced properly on Monday, where there could be a proper statement and a proper explanation about what has gone on."

These are matters of the greatest sensitivity and this must be very very difficult for families to handle. All of us were unaware that this material existed.

Owen Paterson

Addressing the issue of a review, Mr Cameron said he was sure the Secretary of State for Northern Ireland would listen carefully to such requests.

"But let's first of all publish the information on Monday, so that everyone can see what went wrong and why this happened."

Secretary of State Owen Paterson also expressed sympathy to the families who will be shortly told that body parts of their relatives have been kept by police.

"This has been a most difficult revelation and we have to handle it with great, great sensitivity," he said.

Policing Board Chair Brian Rea said the thoughts of the Board members are first and foremost with the families of all those affected by this "highly sensitive review".

He added that questions had been raised "that now require complete openness and transparency" from the PSNI and the other agencies involved.

"This is an issue that is not limited to policing in Northern Ireland and has been subject to review by services across the UK, with a report on the matter due to be published by the Association of Chief Police Officers," he said.