An Iraqi taxi driver may have been the source of the discredited claim that Saddam Hussein could unleash weapons of mass destruction within 45 minutes, a Tory MP claimed today.

Adam Holloway, a defence specialist, said MI6 obtained information indirectly from a taxi driver who had overheard two Iraqi military commanders talking about Saddam's weapons.

The 45-minute claim was a key feature of the dossier about Iraq's weapons of mass destruction that was released by Tony Blair in September 2002. Blair published the information to bolster public support for war.

After the war the dossier became hugely controversial when it became clear that some of the information it contained was not true. An inquiry headed by Lord Butler into the use of intelligence in the run-up to the war revealed that MI6 had subsequently accepted that some of its Iraqi sources were unreliable, but his report did not identify who they were.

Holloway, a former Grenadier Guardsman and television journalist who is now a member of the Commons defence committee, made his comments in a report he has written claiming that MI6 always had reservations about some of the information in the dossier but that these reservations were brushed aside when Downing Street was preparing it for publication.

In the report he wrote: "Under pressure from Downing Street to find anything to back up the WMD case, British intelligence was squeezing their agents in Iraq for information. One agent did come up with something: the '45 minutes' or something about missiles allegedly discussed in a high level Iraqi political meeting.

"But the provenance of this information was never questioned in detail until after the Iraq invasion, when it became apparent that something was wrong. In the end it turned out that the information was not credible, it had originated from an émigré taxi driver on the Iraqi-Jordanian border, who had remembered an overheard a conversation in the back of his cab a full two years earlier.

"Indeed, in the intelligence analyst's footnote to the report, it was flagged up that part of the report probably describing some missiles that the Iraqi government allegedly possessed was demonstrably untrue. They verifiably did not exist.

"The footnote said it in black and white ink. Despite this glaring factual inaccuracy, which under normal circumstances would have caused the reliability of the intelligence to be seriously questioned, the report was treated as reliable and went on to become one of the central planks of the dodgy dossier."

The report was published today on the first defence website. Holloway told the Guardian that he had not seen the intelligence report himself but that he had been told about it by two reliable sources. He said that, although he was not clear whether the footnote related to suspect information about 45 minutes or about missiles or both, he was "100% certain" that it existed.

Sir John Chilcot, the chairman of the Iraq inquiry, said at the opening of this afternoon's hearing that the Holloway allegations might be relevant to his investigation but that he would not be asking about them when he took evidence today from Sir John Scarlett, the former chairman of the Joint Intelligence Committee.

The September dossier did not specify what weapons Iraq could deploy within 45 minutes. Intelligence officials subsequently revealed that it was meant to be a reference to battlefield weapons, not long-range missiles.

But, when it was published, some British papers interpreted the dossier as meaning that British troops based in Cyprus would be vulnerable to an Iraqi attack. At the time the government did not do anything to correct this error.