Britain's former spy chief, Sir John Scarlett, misled the Iraq inquiry by exaggerating the reliability of crucial claims about Saddam Hussein's ability to launch weapons of mass destruction, according to the leading Ministry of Defence expert who assessed the intelligence behind the decision to go to war.

Scarlett who was responsible for drafting the Government's controversial 2002 dossier outlining the case for invading Iraq, claimed last week that intelligence indicating Iraq possessed missiles that could be launched within 45 minutes was “reliable and authoritative”.

But Scarlett's evidence is contradicted by the most senior WMD analyst who saw the original intelligence. Brian Jones said that it was vague, inconclusive and unreliable.

Dr Jones, who was head of the nuclear, chemical and biological branch of the Defence Intelligence Staff in the run-up to the invasion, said that it was “absolutely clear” the intelligence the Government relied upon was coming from untried sources. The 45-minute claim was one of the key assertions that convinced MPs to take Britain to war.

“Having said there was the intelligence to show Iraq had WMD, there was no indication in what (Scarlett) said about what is now very well known, that those additional pieces of new intelligence were all caveated,” said Dr Jones.

He added that Scarlett crucially misled the inquiry about the source of the information. “The description Scarlett gave for the secondary source, who passed the information on, was ‘reliable and authoritative'. If he is passing on information from someone who has never reported before, then that is a nonsense.”

All witnesses to the Iraq inquiry are made to sign a written transcript of their evidence, declaring that it is “truthful, fair and accurate”.

Scarlett was the head of the Joint Intelligence Committee when he oversaw the drafting of the September 2002 dossier.

Despite the controversy, Scarlett was promoted to become the head of MI6 in 2004 and later received a knighthood.

Dr Jones's comments will add to the pressure on Mr Blair ahead of the former Prime Minister's own expected appearance before the Chilcot inquiry panel next month.