Image copyright PA Image caption The inquiry, led by Sir John Chilcot, has been running for nearly three years

The Iraq Inquiry says it cannot proceed with the next phase of its work because key information, including correspondence between Tony Blair and George W Bush, has yet to be released.

The inquiry chaired by Sir John Chilcot had hoped to begin contacting those likely to be criticised in its report this autumn, to allow them to respond.

But it said it had not yet agreed with the government over the publication of the most "difficult documents".

The inquiry began its work in 2009.

In a statement on the inquiry's website, Sir John said the next phase of its work was "dependent on the satisfactory completion of discussions between the inquiry and the government on disclosure of material that the inquiry wishes to include in its report or publish alongside it".

He added: "Since June this year the inquiry has submitted 10 requests covering some 200 cabinet-level discussions, 25 notes from Mr Blair to President Bush and more than 130 records of conversations between either Mr Blair or Mr [Gordon] Brown and President Bush.

"The inquiry secretariat has responded to a number of Cabinet Office questions on those requests, but the government and the inquiry have not reached a final position on the disclosure of these more difficult categories of document."

'Scale of task'

Sir John has written to Prime Minister David Cameron to express his regret that no agreement has yet been reached.

In his reply, the prime minister acknowledged the progress that had been made and said he was "aware of the scale of the task declassification has presented to a number of government departments".

He added: "I appreciate consideration of the disclosure requests for the remaining sensitive categories of information must be handled sensitively and carefully but I hope that consideration of the final sets of papers can be concluded as soon as possible."

A spokeswoman for No 10 said she had nothing to add to the prime minister's letter. The Cabinet Office has released a statement saying "discussions are continuing between the government and the inquiry about the disclosure of records".

BBC political correspondent Carole Walker said this was the latest in a series of delays to the long-running inquiry.

The inquiry, which is examining the background to the UK's involvement in the 2003 invasion and its aftermath, has never set a firm deadline for publishing its final report - set to be about a million words long.

However, it was initially expected to be published in 2012.