MoD scientists knew no ricin had been found at a north London flat raided over an alleged terror plot for weeks before telling police, it has emerged.
Kamel Bourgass was jailed for life in June 2004
Fears the deadly substance was in the Wood Green flat were raised during a raid on 7 January 2003 but disproved by scientists two weeks later.
But it was not until 20 March that police and ministers learned no ricin had been found, the MoD confirmed.
The MoD said the outcome of the April 2005 ricin trial was not affected.
A Ministry of Defence spokesman said the delay in informing police had been caused by a breakdown in communications.
He said the Defence Science and Technology Laboratory (DSTL) at Porton Down in Wiltshire was examining its procedures to make sure similar mistakes did not occur in the future.
Initial tests carried out by DSTL scientists during the 2003 raid indicated there might be ricin in the flat.
This information was released to the press, although the DSTL stresses it at no point said the poison had definitely been found.
It was only after further "gold standard" tests had been carried out back at Porton Down that it became clear there was no ricin.
According to a Porton Down spokeswoman, scientists wrongly believed anti-terror officers were briefed on the later findings straight away.
The test results were eventually communicated to police and ministers, but not before the alleged ricin plot had featured in speeches given by then US Secretary of State Colin Powell as the US and UK made their case for going to war in Iraq.
Evidence gathered in the flat raid was eventually used in the prosecution of Kamel Bourgass, 31, and investigation of eight other men over a bid to poison Londoners with ricin and other substances.
Bourgass was convicted and jailed for 17 years.
Four of the others were tried and found not guilty of involvement. The remaining four were cleared after the prosecution offered no evidence against them.
The DSTL stresses the delay in passing on the final test results to police had no bearing on the outcome of the trial, as this centred on the intention to produce ricin rather than possession of the deadly substance.
Liberal Democrat defence spokesman Michael Moore called for the defence select committee to investigate the delay.
"It is staggering that such a crucial piece of information did not get to the proper authorities, not least when it became so central to the political justification for the war in Iraq," he said.