Sensitive extracts of exchanges between Tony Blair and George Bush in the run-up to the Iraq war are to be published - but the full versions will remain secret.
Years of negotiations over the release of the "vital" material, which includes 25 notes from Mr Blair to Mr Bush and more than 130 records of conversations between the former Prime Minister and then US president, is understood to have been behind the delay in publication of the long-awaited report into the invasion.
Under the agreement that has now been thrashed out, the information being disclosed will be limited to "quotes or gists" and the inquiry's use of the material "should not reflect president Bush's views".
The deal was struck by the Government's top civil servant Sir Jeremy Heywood, who was principal private secretary to Mr Blair in 10 Downing Street in the run-up to the war, and inquiry chairman Sir John Chilcot.
In a letter, Sir John told the Cabinet Secretary: "I appreciate that the inquiry's requests for disclosure raised difficult issues of long-standing principle, which have taken some months to resolve: recognition of the wholly exceptional nature of this Inquiry has allowed that to happen.
"My colleagues and I judge that this material is vital to the public understanding of the inquiry's conclusions."
He added: "Following our recent agreement on the principles for disclosure of material describing communication between the prime minister and the president of the United States, detailed consideration of the gists and quotes requested by the inquiry has now begun.
"Consideration will be based on the principle that our use of this material should not reflect president Bush's views.
"We have also agreed that the use of direct quotation from the documents should be the minimum necessary to enable the inquiry to articulate its conclusions."
Sir John said some "potential gaps" in material had been identified which had now been addressed, "including some material received by the inquiry very recently".
The inquiry completed public hearings in 2011 and today's announcement comes amid mounting criticism over the delay in releasing the report. Officials said the date of publication is yet to be agreed.
Earlier this week, Mr Blair insisted he was not the reason for the delay.
"It certainly isn't me who is holding it up," he told the BBC. "The sooner it is published the better from my perspective as it allows me to go and make the arguments."
Letters must be sent out to individuals facing criticism in the report before it is published under the "Maxwellisation" process to give them an opportunity to respond.
Once that is complete, the report can be finalised and sent to Prime Minister David Cameron, who has previously said he hopes it will be published by the end of the year.
A Cabinet Office spokesman said: " The Government is pleased that agreement on a way forward on both Cabinet papers and UK/US exchanges has now been reached with the inquiry.
"This allows for the declassification and publication of the material the inquiry believes it needs to explain its conclusions.
"Resolving this issue has taken longer than originally hoped but these are sensitive issues.
" The UK/US Head of Government channel is very important and must be handled sensitively.
"The Government and the inquiry are working to ensure the inquiry's report is published as soon as possible and the Government is doing everything it can to facilitate that."