Scotland Yard has reopened the prospect of criminal charges over the death riddle of MI6 spy Gareth Williams.

Lawyers for the force tried to block a coroner from publishing video footage which could be key to a prosecution. Investigations into the discovery of Mr Williams's body in a sports holdall in his bath had drawn a 21-month blank for detectives.

But speaking at the opening of an inquest into his death, lawyer Vincent Williams said charges were still a "real possibility". He warned a "careful line must be struck between open justice" at the inquest and a criminal investigation.

When asked why materials should not go into the public domain, the lawyer added: "It is because there is a live complex ongoing investigation taking place. It is because there may be criminal proceedings further down the line that the Commissioner feels that the pattern of disclosure... has to be done with some care."

Four intelligence agents were given "public interest" anonymity as Coroner Fiona Wilcox said the death was "highly controversial".

Dr Wilcox said "there will be a real risk of harm" to national security and international relations if some of those giving evidence about him are exposed. The only confirmed member of GCHQ who can be named is Stephen Gale, Dr Wilcox said.

The naked and decomposing body of Mr Williams, 31, of Anglesey, North Wales, was found in the bath of his home in Pimlico, central London, in August 2010. He was curled up in a large North Face holdall, sealed by a padlock. The discovery sparked a painstaking investigation, worldwide media frenzy and several outlandish conspiracy theories.

Evidence began with Dr Wilcox expressing sympathy to family members, who fear "some agency specialising in the dark arts" leaves them with no way of knowing how and why he died.

Mr Williams' family object to the release to the media of a video of an expert trying to lock himself in the bag in which the spy's body was found.

The coroner suggested that the expert, Peter Faulding, could have his face pixelated out. But Mr O'Toole expressed concerns that it would be "upsetting" for Mr Williams' relatives if the "speculative" film was released more widely. He said: "The family are anxious to ensure that that reconstruction does not become bandied about the various media outlets, both in this country and abroad."