See The Death of David Kelly - The Fictitious Mobile Weapons Laboratories.
I've now found a contemporary report, from the Observer of 20th July 2003, confirming that it was DAvid Kelly who told the Observer that the supposed mobile weapons labs were bogus:
In one particular Kelly was definitively at odds with what both George Bush and Blair were claiming about postwar discoveries of Iraqi weapons. Both had insisted that Coalition Forces had found and identified two alleged mobile germ warfare laboratories. But Kelly had a totally different opinion, telling The Observer that he had examined the alleged labs in person and had no doubts about what they were intended for - the production of hydrogen to fill artillery balloons. 'Exactly as the Iraqis described them,' he would tell this paper.
See A haunted man for the source of the preceding quote.
Just how much did David Kelly's honesty on that point anger very powerful people?