Sir John Chilcot is facing legal action from bereaved families after again defying calls to set a timetable for publication of the Iraq Inquiry report.
The chairman said he understood the "anguish" of those who lost loved ones in the conflict, but argued that the probe was "unprecedented" in its scope.
He also defended the controversial Maxwellisation process, which means the inquiry seeks responses from everyone facing criticism before its conclusions are published.
The statement came amid renewed pressure to explain why the report has yet to emerge, six years after it was first commissioned by former Prime Minister Gordon Brown.
Sir John said the inquiry expected to receive the last Maxwellisation responses "shortly" and would then be able to produce a timetable for the report.
However, Sir John's justification was immediately rejected by bereaved families, who have been threatening a judicial review to force him to publish.
Matthew Jury, of McCue & Partners, a law firm representing some of the families, said: "Sir John has refused the families' legal request to impose an effective timetable on the Maxwellisation Process, thereby preventing any deadline being set for publication of the report.
"All he has stated is that witnesses 'have not been given an open-ended timescale' to respond to criticisms.
"By itself this is meaningless, and the Inquiry refuses to disclose to the families anything that would explain it further.
"In the circumstances, the families' legal team, which includes Lord Brennan QC and Sarah Hannett of Matrix Chambers, will be taking further legal steps as are considered necessary and appropriate."