Tony Blair is on his way to Washington for talks with US President George Bush on Iraq, shortly after conceding that the coalition was not winning the war in the country.
His visit coincides with the publication of the long-awaited report by James Baker's bi-partisan Iraq Study Group, which is expected to call for a major shift in US strategy on the war.
Leaks have suggested the report would urge Mr Bush to pull US troops out of combat roles by 2008 and recommend that tens of thousands of non-combat forces should instead stay to advise, train and embed with Iraqi services.
The United States faces a "grave and deteriorating" situation after nearly four years of war in Iraq, it is expected to warn.
Mr Bush received an advance copy of the report and said he would consider its recommendations "seriously" and act "in a timely fashion". But he added: "We probably won't agree with every proposal."
Mr Blair is expected to be in the air over the Atlantic at the time of the report's official publication, and will receive a copy when he lands in Washington.
At Prime Minister's Questions in the House of Commons, he was challenged by Tory leader David Cameron over remarks made on Tuesday by Mr Bush's nominee for Defence Secretary, Robert Gates.
"He said we were not winning the war in Iraq. Do you share that very serious assessment?" demanded Mr Cameron.
Mr Blair replied: "Of course. In July, I said myself that the situation in Baghdad with sectarian killing was appalling and the bloodshed was appalling.
"What is important, however, is, as he went on to say, that we do go on to succeed in the mission that we have set ourselves."
