British code breaker Gareth Williams

Secret agents specialising in the “dark arts” may have killed an MI6 spy who was found locked inside a bag, a coroner heard today.

Relatives believe a third party was either present when Gareth Williams died or broke into his home afterwards to destroy evidence, lawyer Anthony O’Toole said.

The family is demanding answers after Scotland Yard revealed a key line of its inquiries had been an 18-month DNA mix-up.

Mr O’Toole told a pre-inquest review that Westminster Coroner’s Court must establish why there was no evidence of another person in his London apartment when he died.

He said: “The impression of the family is that the unknown third party was a member of some agency specialising in the dark arts of the secret services - or evidence has been removed post-mortem by experts in the dark arts.”

Forensic scientists apologised to the family of holdall spy Gareth Williams today after it emerged that a DNA error had dashed a “key” line of inquiry into his death.

LGC said one of its staff members made a “typographical error” while inputting code, leading Scotland Yard to spend more than a year trying to trace a non-existent suspect.

It was confirmed just last month, when LGC carried out a review, that the partial DNA profile belonged to a scientist involved in the case.

“Having made further checks, LGC identified the partial profile as matching that of a Metropolitan Police scientist who was involved in the original investigation of Mr Williams’ home,” a LGC spokeswoman said.

“The Metropolitan Police Service was immediately notified.

“We are sorry for any pain this error may have caused Mr Williams’ family.”