Spy chief Sir John Scarlett is being questioned about intelligence on Iraq after claims a taxi driver was the source for the most infamous claim.
The man who drew up the September 2002 dossier, which said Iraq could deploy weapons of mass destruction within 45 minutes, is facing the Chilcot inquiry.
His appearance came after Tory MP Adam Holloway said advice casting doubt on the 45-minute claim was “ignored”.
The inquiry chairman said it would deal with the MP’s claim where “relevant”.
However, Sir John Chilcot said it was not specifically a “matter” for Tuesday’s inquiry hearing.
Sir John Chilcot said the former chairman of the Joint Intelligence Committee (JIC) would be asked whether the intelligence community was asking and providing answers to “crucial questions” about the threat from Iraq before and during the conflict.
Ministers withdrew the 45-minute claim after the war but said they acted in good faith.
It was at the heart of the row between the government and the BBC which culminated in the death of government weapons expert Dr David Kelly.