The British inquiry into the 2003 Iraq war and its aftermath, which completed its last hearing in February 2011 with the promise to report back in “some months”, is unlikely to be published this year.

BBC Newsnight said that protracted correspondence with those criticised in the document was one reason that its publication in 2015 looked “doubtful”.

The latest delay, if confirmed, will come as a severe blow to families of the 179 British servicemen killed in the nine-year war. It will also compound public scepticism. Earlier this year its chairman Sir John Chilcot said that the report would not be published before the election. Many saw the full parliamentary term of the new government in October and November as the next plausible deadline window.

The inquiry was set up in 2009 by the Prime Minister at the time, Gordon Brown, and took public evidence from its last witness in February 2011. The latest delay is understood to be due to the so-called "Maxwellisation" process, under which witnesses are sent draft sections criticising them to give them a final right of reply.

Letters were reportedly sent to those cited last December and are understood to run into hundreds of pages with a specific deadline. But the BBC suggests those have been challenged, moved or even abandoned altogether.

The Iraq War: A timeline Show all 16 1 /16 The Iraq War: A timeline The Iraq War: A timeline 11 September 2001 Terrorists belonging to al-Qaeda use hijacked aeroplanes to kill 2,996 people in attacks on the east coast of the US. AP The Iraq War: A timeline 12 September 2001 Tony Blair promises George W Bush that the UK will support the US, whatever the President decides to do. AFP/Getty Images The Iraq War: A timeline 25 March 2002 Jack Straw, Foreign Secretary, warns Blair that invading Iraq would be legally dubious. Getty Images The Iraq War: A timeline June 2002 Tony Blair asks defence officials to outline options for UK participation in military action against Iraq. afp/getty images The Iraq War: A timeline 24 September 2002 The government publishes a dossier about the threat from Iraq’s weapons of mass destruction. A foreword by Tony Blair states that Saddam Hussein’s “military planning allows for some of the WMD to be ready within 45 minutes of an order to use them”. It is subsequently alleged that this dossier was “sexed up” for political reasons. Getty The Iraq War: A timeline 2 October 2002 Congress authorises President Bush to use military force against Iraq. Getty The Iraq War: A timeline 8 November 2002 UN Security Council passes resolution 1441, insisting that weapons inspectors be allowed back into Iraq and calling on the regime to give up its WMD or face the consequences. Simon Walker/AP The Iraq War: A timeline 18 July 2003 David Kelly, an expert in biological warfare, is found dead after being named as the source of quotations used by the BBC’s Andrew Gilligan to suggest that the dossier of September 2002 had been “sexed up”. Lord Hutton is appointed to chair a judicial inquiry into his death. GETTY IMAGES The Iraq War: A timeline 13 December 2003 Saddam Hussein is captured near Tikrit, after nine months in hiding. REUTERS The Iraq War: A timeline 2 March 2004 Bombings in Baghdad and Karbala kill nearly 200 people: the worst attacks since the fall of Saddam. Getty Images The Iraq War: A timeline 14 September 2005 Bombs in Baghdad kill 160 people and injure more than 500. EPA The Iraq War: A timeline 30 December 2005 Saddam Hussein is executed. Getty Images The Iraq War: A timeline 28 May 2009 The last British combat troops leave Iraq. Getty Images The Iraq War: A timeline 24 November 2009 The Chilcot inquiry holds its first public hearing. Getty The Iraq War: A timeline 2 February 2011 The Chilcot inquiry holds its final public hearing. AFP/Getty Images The Iraq War: A timeline 21 January 2015 Sir John Chilcot confirms that his report will not be published before the general election in May 2015. Getty Images

“Those I've spoken to in recent months say the Maxwellisation process has become a nightmare,” BBC Newsnight’s Diplomatic and Defence Editor Mark Urban wrote on Monday.

“Some of the dozens criticised have had to sift through Chilcot inquiry drafts that run to hundreds of pages. Others have engaged in a lively correspondence with the inquiry team, asking to draw on all sorts of additional correspondence in their defence.”

That reasoning chimes with testimony given to MPs by Sir John in February. When explaining the reason for its delay he said: “What I can't say, until the Maxwellisation process is complete, is that I will be able to say anything useful to the Prime Minister or to the families. Once that is complete it is a different matter."

In his blog, published on the BBC website on Monday, Mr Urban added: “While Sir John originally intended to give those due to be censured a deadline by which they would have to make representations in their defence, these have apparently been abandoned as the complexity of the process has become clear.

“The inquiry has also suffered from the death of one of its members and staff losses.”

Previous delays in publication have been down to protracted disagreements between Whitehall and the US State Department over declassifying communications between George W Bush and Tony Blair before, during and after the Iraq war. At the start of 2015 the Government admitted that talks over the publication of the gist of those conversations were now complete.

The report is understood to be over a million words long, analyses over 150,000 government documents and will contain details of discussions of more than 200 cabinet meetings.

David Cameron has previously said he has no control over when the Chilcot report will be published.