An influential parliamentary committee has raised concerns over the lengthy delay in publication of the Chilcot report into the Iraq War.
Sir John Chilcot's inquiry completed public hearings in 2011, but publication of its report is understood to have been held back by negotiations over the publication of private communications between Tony Blair, prime minister at the time of the 2003 conflict, and then US president George Bush.
The House of Commons Public Administration Committee described the delay as "very serious" and its chairman, Bernard Jenkin, has written to the Cabinet Office demanding an explanation for the hold-up.
He made clear that he is ready to summon ministers, and Cabinet Secretary Sir Jeremy Heywood, for questioning on the reason why the report has not yet been published.
In an interview to be broadcast on BBC2's Daily Politics today, Mr Jenkin said: "It's very serious that this report is now at least four years overdue, so we've written to the minister to ask for an explanation as to why these delays have occurred, what is holding up the publication of the report and how these issues are going to be resolved.
"On the basis of that we may well call for the minister, or indeed the Cabinet Secretary, to come and give us evidence to explain how they are going to sort this out."
Following the completion of his inquiry, Sir John began a process known as "Maxwellisation" under which individuals facing criticism in the report are given an opportunity to respond before publication.
In a letter to Prime Minister David Cameron last year, he said he was in discussions with Sir Jeremy - the Government's most senior civil servant, who was principal private secretary to Mr Blair in 10 Downing Street in the run-up to the war - over his plan to publish sensitive material, covering some 200 Cabinet-level discussions, 25 notes from Mr Blair to Mr Bush and more than 130 records of conversations between the PM and the US president.
Radio 4's Today programme reported that the Cabinet Office s aid there had been constructive dialogue with the inquiry and the process would be concluded as quickly as possible.