The Prime Minister has urged the chairman of the Iraq Inquiry to speed up publication of his report after it was announced it would not be released until next summer.

David Cameron said he was "disappointed" after Sir John Chilcot wrote to inform him he would not be ready to publish the much-delayed Iraq Inquiry report until June or July.

In a letter to the Prime Minister , Sir John says the "text of the report" will be ready by April but it will not be ready for publication until a date in June or July 2016.

The report, which will have taken seven years to complete and will run to two million words - nearly four times the length of War and Peace - has suffered significant delays and cost the taxpayer £10.3m, which works out at around £5.15 a word.

In a letter responding to Sir John, Mr Cameron says he was "disappointed" by the delay and said the families of those who served in Iraq would be disappointed too.

He tells Sir John he would "welcome any further steps you can take to expedite the final stages of the inquiry" and offers extra resources to help speed up the process.

Sir John explains in his comments to the Prime Minister that the sheer scale of the report and the work needed to check it and prepare it for printing will take a number of months.

He wrote: "I entirely understand that a checking process is necessary and is normal procedure in Inquiries which have considered a large volume of sensitive material" to ensure that the Government meets its obligations under Article 2 of the European Convention on Human Rights and for the protection of National Security.

Sir John also explained: "The very considerable size of our report - more than two million words in total - means that it will take some weeks to prepare for printing and publication ... We will complete that work as swiftly as possible. 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."

Sir John ended his letter: "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."

Rose Gentle, whose Royal Highland Fusilier son Gordon, 19, was killed in a bomb attack in Basra in 2004, said she was "disappointed" by the news.

Ms 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."

The report has been significantly delayed by the process of "Maxwellisation" whereby those criticised in the report are given a right to respond.

Tony Blair, who ordered the Iraq invasion in 2003, has consistently denied he was responsible for delays to the inquiry reporting back.

Responding to the announcement, a statement said: "Tony Blair has always wanted the Inquiry to report as soon as it properly can and he looks forward to responding to the Inquiry's report.

"Mr Blair also wants to make it clear that the timetable of the inquiry and the length of time it will have taken to report is not the result either of issues over the correspondence between him as prime minister and President Bush; or due to the Maxwellisation process. "

At the weekend the former Labour prime minister apologised for "mistakes" in planning the military intervention in Iraq saying that the "intelligence we received was wrong".

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

"I also apologise for some of the mistakes in planning and, certainly, our mistake in our understanding of what would happen once you removed the regime."

It was widely reviewed as an attempt to limit some of the fallout from the publication of the Chilcot report.

The current Labour leader Jeremy Corbyn had vowed to publicly apologise on behalf of the party for leading Britain to war in Iraq when he became leader.

However, Labour sources say he will now wait to apologise until Sir John has published his report.

He also voiced concerns about the delay in publication and said: "I hope it is the whole story, I hope stuff hasn't been left out of it, I hope we're not still negotiating with George Bush and Tony Blair about what's in the report.

"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."

The Iraq Inquiry was ordered by Gordon Brown in 2009 and heard from its last witness in February 2011. Mr Blair appeared before the inquiry twice.