The author of the inquiry into the Iraq War is facing criticism after announcing his report would not be published until June or July 2016.
The United Kingdom's Prime Minister David Cameron said he was "immensely frustrated" at how long Sir John Chilcot's inquiry, which began in 2009, was taking.
The mother of a British soldier killed in Iraq said it was "another let-down".
Sir John said the two million-word report would be finished in April and then given national security checks.
Mr Cameron offered resources to speed up the process, while Labour leader Jeremy Corbyn said the time taken was "getting beyond ridiculous".
The inquiry is considering how UK forces came to participate in the US-led invasion of Iraq in 2003 and its aftermath.
The new publication timetable was set out in a letter to Mr Cameron on the inquiry's website.
Sir John had previously rejected calls to publish a timetable for publication, saying he did not want to "arouse false hopes".
But in his letter, Sir John says the text of his report should be completed in the week starting 18 April 2016, at which point the process of national security checking would begin.
Such checking is "normal and necessary" with inquiries handling large amounts of sensitive material, he said.
It will ensure that national security and Article 2 of the European Convention on Human Rights, which protects the right to life, are not "inadvertently breached" by publication, he said.
"I consider that once national security checking has been completed it should be possible to agree a date for publication in June or July 2016," he added.
Sir John ended his letter to the Prime Minister saying: "My colleagues and I remain committed to producing a report that will meet the very wide ranging terms of reference we were given and reflect the considerable investment of time and effort by all involved."
In his letter back to Sir John, Mr Cameron welcomed the fact there was "a clear end in sight" but added: "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 said he would welcome measures to "expedite" the final stages, saying the government would be "very happy" to provide more resources if it meant the inquiry was published sooner.
The Prime Minister said the government intended for the national security checking to last no longer than two weeks.
Speaking later at a summit in Iceland, the Prime Minister said he was "immensely frustrated by the slowness and the amount of time that it has taken".
He added: "And I'm not frustrated on my own behalf, I'm frustrated for the mums and the dads who lost loved ones and who want to know what happened and why it happened and want to make sure that the lessons are learnt."
Labour leader Jeremy Corbyn said: "We need to know what happened, we need to know why it happened, we need to know who made the decisions and we never need to make these kind of catastrophic mistakes again."
Addressing MPs in the House of Commons, Speaker John Bercow said there was "a very real sense of anger and frustration across the whole House at what seems a substantial disservice that has been done".
I'm not responsible for delays - Blair
A statement from former Prime Minister Tony Blair's office said he looked forward to responding to the report.
He has repeatedly stressed he was not the reason for the drawn-out timescale, a point he repeated in his statement.
Any delays were "not the result either of issues over the correspondence between him as Prime Minister and President Bush; or due to the Maxwellisation process," his office said.
Mr Blair and other witnesses had been given information under Maxwellisation "very late in the process", the statement said, adding that suggestions he had been the cause of delays in publication were "categorically incorrect".
Reg Keys, whose son, Lance Corporal Tom Keys, was killed in Iraq in June 2003, was critical of the "ridiculous" Maxwellisation process, saying it had been allowed to "run on far too long".
He predicted the report, when it was eventually published, would be a "watered-down" version of criticism raised during the inquiry.
The mother of Royal Highland Fusilier Gordon Gentle, who was killed aged 19 in a bomb attack in Basra in 2004, said she was "disappointed" by the latest news from the inquiry.
Rose Gentle, from Glasgow, said: "We thought it should be out a lot sooner than this. I thought it would be out by the end of the year, because they have everything there.
"It's another let-down. It's another few months to wait and suffer again."
Background to today's development
- The US-led invasion of Iraq started on 19 March 2003 with a "shock-and-awe" campaign intended as a show of force
- The US and the UK claimed Iraqi leader Saddam Hussein had weapons of mass destruction he was capable of using
- The capital Baghdad fell in April and US President George W Bush declared "mission accomplished" weeks later
- Saddam Hussein was captured, tried by the new Iraqi government and hanged. Insurgency continued
- British forces ended combat operations in 2009 and the US did so the following year
- A total of 179 UK service personnel and nearly 4,500 US soldiers were killed in the conflict
- British-based organisation Iraq Body Count estimates 134,400 to 151,652 Iraqi civilians died since 2003, and United Nations estimates 18,805 between 2008-12 - all counts and estimates of Iraqi deaths are highly disputed
- The Chilcot inquiry into the UK's role in the war was established by Prime Minister Gordon Brown in 2009