The report of an inquiry into Britain's role in the Iraq war will be delayed until after a national election in May, its chairman said, provoking dismay from politicians of all parties Wednesday.

Sir John Chilcot, a former senior civil servant who is heading an inquiry that began in 2009, wrote to Prime Minister David Cameron saying that the report was being delayed because individuals mentioned in it need a chance to respond.

"Until we have received and evaluated responses from all those who have been given the opportunity to respond, I cannot give an accurate estimate for how long it will then take to complete our work, but it is still clear that will take some further months," Chilcot said in the letter, released Wednesday.

"I therefore see no realistic prospect of delivering our report to you before the General Election in May 2015."

Britain's participation in the U.S.-led invasion was deeply controversial at the time, and became more so when there proved to be none of the weapons of mass destruction which had been the key argument for deposing Iraqi dictator Saddam Hussein.

The inquiry had its last evidence session in 2011, but the release has been stalled by wrangling over the inclusion of classified material — including conversations between then-Prime Minister Tony Blair and President George W. Bush.

Cameron expressed dismay at the decision, but rejected suggestions that the delay might make it appear there was a cover-up.

"I don't think that's right at all," Cameron said.

Deputy Prime Minister Nick Clegg said "the public have waited long enough" and that further delay in publication could undermine public trust in its conclusions.

He said there is a danger the public will assume the report's conclusions will be muted by individuals responding to criticisms of their roles.

When then-Prime Minister Gordon Brown established the inquiry in 2009, it was planned to be completed within a year.