The Chilcot inquiry, which is expected to contain damning criticism of the way Tony Blair and his close advisers led Britain into war against Iraq, is unlikely to be published until next year, the Guardian has learned.

A further delay in the report on the 2003 invasion, due to have been published three years ago, could mean the issue will continue to haunt British politics in the runup to next year's general election.

That is likely to be even more the case if there is no end in sight to the present crisis in Iraq which threatens to perpetuate deep divisions and violence in the oil-rich country.

Whitehall sources suggest the latest delay in the long-awaited report is the result of continuing disputes over criticisms the Chilcot panel plan to make of Blair and other ministers and advisers involved in the decision to invade Iraq.

Chilcot announced last month that after years of heated disputes with successive cabinet secretaries, and discussions with Washington, he had agreed to a settlement whereby summaries, and "the gist", of more than a hundred records of conversations between Blair and George Bush in the runup to the invasion, and of records of 200 cabinet discussions, would be published, but not the documents themselves.

Chilcot has described the content of the documents as "vital to the public understanding of the inquiry's conclusions".

In a letter to Sir Jeremy Heywood, the cabinet secretary, last month, Chilcot said "detailed consideration" of the information he has requested had begun, adding "it is not yet clear how long that will take".

In a reference to the procedure taken from the inquiry into Robert Maxwell's business dealings, whereby those the inquiry intends to criticise will be shown drafts of the relevant passages, Chilcot told Heywood that "once agreement has been reached, the next phase of the Maxwellisation process can begin".

Philippe Sands QC, professor of law at University College London, said: "How painfully ironic that Britain used force in 2003 when it was manifestly illegal, but will likely and rightly not do so now in response to a request from the government of Iraq, when it would rather more arguably be lawful."

Sands, a close follower of Chilcot and earlier inquiries into the invasion of Iraq, added: "The situation in Iraq today is terrible and tragic, but it's a futile exercise to speculate as to the exact connection with decisions taken in 2003 … It would be more sensible to reflect on what might be learnt from the mistakes of the past."

He continued: "Who exactly is responsible for the delay [in the Chilcot report] is unclear, but it is hard to avoid the suspicion that political considerations might have come into play."

Sands described the delay as "rather disgraceful", and said publication of the report should now be delayed until after the election, to avoid it being used as a political tool.

David Cameron said last month he wanted the report by the end of the year. The Scottish National party has said it should be published before September's referendum on Scottish independence.

The Chilcot inquiry was set up in 2009 and has cost over £9m so far.