Prime Minister David Cameron on Thursday said he was "disappointed" that a long-awaited report into Britain's role into the Iraq War -- initially due to have been published in 2010 -- would not be released until next June or July.

In a letter to Cameron published Thursday, inquiry chairman John Chilcot said the two-million-word report would be completed by mid-April 2016, ready for the authorities to conduct "national security checking".

"I consider that once national security checking has been completed it should be possible to agree with you a date for publication in June or July 2016," he said.

In his reply, Cameron welcomed the setting out of a timetable for the report's publication, but criticised its contents.

"I am disappointed -- and I know the families of those who served in Iraq will also be disappointed -- that you do not believe it will be possible logistically to publish your report until early summer," he wrote.

"I recognise that you have a significant task, but would welcome any further steps you can take to expedite the final stages of the inquiry."

Cameron also said that the government aimed to complete the security checks within two weeks.

The Chilcot inquiry was set up in 2009 by prime minister Gordon Brown -- the successor of Tony Blair, who led Britain into the 2003 conflict -- and was originally due to report within a year.

Some 179 British soldiers died in the war.

The report is expected to highlight how Britain's involvement in Iraq -- particularly questions over whether Blair's government "sexed up" a dossier on Iraq's weapons of mass destruction to make the case for war -- remains the subject of heated debate.

Blair had previously denied that the delay was linked to a process in which individual witnesses such as himself are given the chance to respond to criticism of them in the report.

"For people like myself, when this inquiry finally comes out, it gives me also frankly an opportunity to come out and say 'this is why I did what I did', 'this is why I believed it was the right thing to do'," he told the BBC.

Blair apologised on Sunday for intelligence failings and planning mistakes around the invasion but insisted he still did not regret the removal of Saddam Hussein as Iraq's leader.

"I apologise for the fact that the intelligence we received was wrong," he told CNN in an interview.

Relatives of those killed in the conflict said they were disappointed by Thursday's announcements.

"We thought it should be out a lot sooner than this," said Rose Gentle, whose son Gordon, 19, was killed in a bomb attack in Basra in 2004.

"It's another let-down. It's another few months to wait and suffer again."

Reg Keys, father of Lance Corporal Tom Keys, who was killed in 2003, told the BBC he felt "a certain amount of anger because I just think this delay has gone on just far too long now".