He believes the fact he has access to all government records will deter people from lying because, as he puts it, "the stuff is there on paper anyway".
"We have complete access to the entirety of the government's records from top to bottom throughout the nine years and we have already seen more than enough to know nothing is being held back because it can't be," he said.
"If there was a gap we would know it and find it."
In the run-up to Christmas, witnesses called will include senior officials, diplomats and military officers.
He said his team had been working its way through an "absolute mountain range" of documentary evidence.
Members have also met the families of most of the 179 British service personnel who lost their lives in the course of the campaign, which began with the US-led invasion of 2003.
Through this work some of the main areas the inquiry will focus on have already become clear, he said.
They included the perennial issue of resources - whether there was enough "manpower and material" - the interaction between political decision-making and military planning, and aftercare for the bereaved and wounded.
But Sir John acknowledged that, for many people, the overriding questions would be whether Britain was right to go to war and whether the conflict had been legal.
Asked if he could provide the definitive answers, he said. "Definitive in one sense, yes, but not definitive in the sense of a court verdict of legal or illegal.
"It is much closer to high policy decisions - was this a wise decision? Was it well-taken? Was it founded on good advice and good information and analysis?" he said.
Sir John said he hoped to finish the report before the end of next year.
He argued it would be irresponsible to publish any interim findings ahead of the General Election, because there was a serious risk they could prove to be incomplete or unsatisfactory and have to be changed.
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