MI6 codebreaker Gareth Williams was probably killed but the "spy in the bag" case might never be solved after mistakes by investigators, an inquest has heard.

Mr Williams's relatives attacked failures by secret services and police after a coroner ruled "many agencies fell short" in their investigation of the holdall death riddle.

Fiona Wilcox said she was sure a third party locked the 31-year-old mathematics prodigy inside the red holdall, probably while he was still alive.

She criticised the 21-month investigation, saying it was unlikely the mystery "will ever be satisfactorily explained".

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

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

As the eight-day hearing ended, relatives spoke out for the first time about their grief being "exacerbated" by MI6's "reluctance and failure" to assist the police inquiry.

In a statement read out by their solicitor, they said they were "extremely disappointed" at "total inadequacies" in the inquiry.

Scotland Yard vowed to explore new evidence that has come to light, while Sir John Sawers, chief of the Secret Intelligence Service, apologised "unreservedly" for delays in raising the alarm about the death.

Mr Williams, a fitness enthusiast originally from Anglesey, North Wales, was found naked, curled up in the padlocked holdall in the bath of his flat in Pimlico, central London, on August 23 2010.