MI6 codebreaker Gareth Williams was probably killed unlawfully but the case might never be solved, a coroner has said.

Fiona Wilcox said she was sure a third party locked Mr Williams inside the red holdall in which he was found dead.

And she criticised the 21-month investigation as she said it was unlikely the riddle "will ever be satisfactorily explained".

"The cause of his death was unnatural and likely to have been criminally mediated," she said in her two-hour narrative verdict at Mr Williams's inquest.

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

Mr Williams's family criticised his employers at MI6 afterwards for their response to his death, saying "our grief was exacerbated" by it.

His sister looked on as Dr Wilcox told a packed Westminster Coroner's Court that "on the balance of evidence" he was probably alive when he was put in the bag.

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

But she said "there was no evidence to support that he died at the hands of" spies as the inquest drew to a close.

She ruled that "it would appear that many agencies fell short" during the inquiries into his death.