Britain's government agreed on Thursday to give extracts of letters from Tony Blair to George W. Bush to an inquiry into the Iraq war, overcoming the main hurdle to publication of the long-awaited report.

The probe will receive "gists and quotes" of communications from former prime minister Blair to ex-president Bush in the run-up to the conflict in 2003, inquiry chief John Chilcot said in an official letter.

But Bush's replies will not be included in the report, which is examining Britain's involvement in the war, Chilcot said.

"I am pleased to record that we have now reached agreement on the principles that will underpin disclosure of... communications between the UK Prime Minister and the President of the United States," Chilcot said in his letter to Jeremy Heywood, the British government's top civil servant.

"These documents have raised difficult issues of long-standing principle," Chilcot wrote.

The inquiry was set up in 2009 and was expected to report in 2010. The last public hearings took place in 2011.

But disagreements over the publication of some 25 written notes from Blair to Bush and more than 130 records of conversations have been the biggest factor in delaying the findings of the inquiry.

Chilcot, himself a former civil servant, said the government had now begun "detailed consideration of gists and quotes requested by the inquiry" and urged it to answer as soon as possible.

"Consideration will be based on the principle that our use of this material should not reflect President Bush's view," he said.

The inquiry chief said the new material was "vital for the public understanding of the inquiry's conclusions."

There had been fears that Blair -- who as Labour prime minister committed British forces to the US-led invasion of Iraq after forming a close bond with then-president Bush -- and the US administration would block the release of the confidential papers.

"The Inquiry intends to submit its report to the Prime Minister as soon as possible," Chilcot added.

He did not give a date but reports earlier this year said it was expected before the end of the year.

Blair, who went on to become a Middle East peace envoy, insisted earlier this week that he was not to blame for the delay in publication of a report which could heavily criticise his handling of the war.

"It certainly isn't me who is holding it up. The sooner it is published the better from my perspective as it allows me to go and make the arguments," he told BBC radio on Tuesday.

British Prime Minster David Cameron has also expressed his hope that the report will be published before year's end.

Britain was the second largest contributor of troops to the Iraq invasion after the United States.

A total of 179 military personnel were killed in Britain's six-year involvement in Iraq.

Blair's relationship with Bush came under close scrutiny in Britain, where opposition to the Iraq war saw around one million people march in London in 2003.

Blair has previously denied that he and Bush, both committed Christians, prayed together before the invasion of Iraq.