Spy death mystery may never be solved

Speaking as she delivered a narrative verdict following a seven-day inquest, Ms Wilcox said: "The cause of his death was unnatural and likely to have been criminally mediated.

"I am therefore satisfied on the balance of probabilities that Gareth was killed unlawfully."

Sir John Sawers, chief of the Secret Intelligence Service, apologised to Mr Williams's family for MI6's failure to report his disappearance for a week.

Sir John expressed the "deepest condolences" of MI6 and the Government Communications Headquarters and said the service should have acted more swiftly when Mr Williams failed to turn up to work in August 2010. The MI6 chief said "lessons have been learned, in particular the responsibility of all staff to report unaccounted staff absences".

However, Mr Williams's family said their "grief was exacerbated" by MI6's actions.

In a statement read out by their solicitor, they said they were disappointed at the secret services' reluctance and failure to make relevant information available to the inquiry.

They also attacked the "total inadequacies" of the inquiry by Metropolitan Police counter-terror branch SO15 into MI6 and called on Scotland Yard's chief to look into how the investigation would proceed.

The statement added: "To lose a son and a brother at any time is a tragedy. To lose a son and a brother in such circumstances that have been outlined in the course of this inquest only compounds the tragedy.

"Our grief is exacerbated by the failure of his employers at MI6 to take even the most basic inquiries as to his whereabouts and welfare, which any reasonable employer would have taken.

"We are also extremely disappointed at the reluctance and failure of MI6 to make available relevant information. Gareth was and always will be a special and adorned son and brother. We miss him every single day and cannot describe the depth of sorrow his absence leaves in our lives."

Earlier, Ms Wilcox told Westminster Coroner's Court that "on the balance of evidence" Mr Williams was probably alive when he was put in the bag.

Ms Wilcox said it remained a legitimate line of inquiry that the secret services were involved in Mr Williams's death.

However, she said there was no evidence to support that he died at the hands of spies. She said "it would appear that many agencies fell short" during the inquiries into his death and "most of the fundamental questions in relation to how Gareth died remain unanswered".

Mr Williams was suffocated by carbon dioxide, possibly as an onset of a short-acting poison, the coroner suggested.

Ms Wilcox said many factors hampered inquiries. Breakdowns in communication by her office in ordering a second post-mortem examination, a DNA mix-up by forensics and the late submission of evidence by MI6 to police were singled out.

Mr Williams, a 31-year-old fitness enthusiast originally from Anglesey, North Wales, was found in the padlocked holdall in the bath of his flat in Pimlico, central London, in August 2010.

Experts said even escapologist Harry Houdini would have struggled to lock himself inside, while Mr Williams's family lawyer suggested the "dark arts" of the secret services were responsible.

Police recovered no evidence of a third party being present and have no suspects.

Dr Wilcox said in her verdict the "unusual circumstances" of Mr Williams's death raised the possibility of foul play.

She said: "I find this did affect the quality of the evidence that was heard before this court."

However, the coroner added that "taking all these shortfalls together, I am satisfied that the evidence is reliable".

Scotland Yard has said it will review lines of inquiry.

Lead detective Jackie Sebire said: "Obviously a lot of information has come out through the course of this inquest which we have not been party to.

"My strongest belief is a third party was involved and I would ask people to search their consciences and come to us to find some resolutions to this case and some peace for his family."