Tony Blair has agreed with the incoming US defence secretary's assessment that the war in Iraq is not being won.
The report is expected to recommend a phased withdrawal
Asked by Tory leader David Cameron at prime minister's questions if he agreed with Robert Gates' assessment of the war, Mr Blair said: "Of course."
Mr Blair has arrived in Washington for discussions with President Bush about the Iraq Study Group's report.
Foreign Secretary Margaret Beckett said the report was "substantial and complex" and in line with UK policy.
The report from the study group, which was set up by Congress and led by ex-Secretary of State James Baker, calls for combat troops to leave Iraq in 18 months.
The report has called the situation in Iraq "grave and deteriorating" and said the current US approach was "not working". It said the ability of the US to influence events was diminishing.
It also said 2,900 American lives had been lost and $400bn (£203bn) spent in the conflict.
On Tuesday, Mr Gates said the next two years could see slow improvements or "the very real risk of a regional conflagration".
In response to Mr Cameron during PMQs, Mr Blair said: "What's important however is that we go on to succeed in the mission we set ourselves."
Mr Cameron said it was essential strategy for Iraq was decided "by the British government and in the British national interest".
Liberal Democrat leader Sir Menzies Campbell asked: "Isn't it clear that the British government has no policy of its own in relation to Iraq? And that we are wholly dependent on the decisions taken in Washington?
"What sort of strategy is that? And what sort of legacy is that?"
'Fight and defeat'
Mr Blair said the British stood side-by-side with the US and the Iraqi government in working to make the country stable.
The prime minister added: "As progressively, the Iraqis are capable of taking on their own security, as now they are doing in two out of the four provinces and in one half of Basra... then the need for British troops diminishes. That is our strategy."
He said that the whole of the Middle East needed to be put on a more stable footing and that part of that meant finding a solution for the Israel-Palestine conflict.
During PMQs, Mr Cameron asked Mr Blair what he believed the changes to US policy following the US report should be.
Mr Blair, who gave evidence to the US inquiry, said: "Inside Iraq, what is important is that we complete the building up of the capability, particularly of the Iraqi army.
"We've also got to make sure that the governance and capability of the Iraqi government is improved - not just in relation to the way it functions but also, for example, to the disbursement of money both in Sunni and Shia areas.
"It's also important we make sure the process of reconciliation ... is carried through, with greater effect than so far."
The Iraq Study Group report also suggested a diplomatic offensive setting up a support group, which would include Iran and Syria.
Mr Blair has raised the possibility of discussion with Iraq's neighbours; something Washington has so far ruled out.
Responding to the publication of the report, Sir Menzies Campbell called the Iraq "adventure" Britain's "biggest foreign policy mistake since Suez".
He said: "The damage to Britain's interests has been incalculable."
For the Conservatives, shadow foreign secretary William Hague, speaking from Pakistan, said they welcomed the report and hoped Mr Blair would make a Commons statement on it before Christmas.
He said: "It seems to make many sensible recommendations, many of which stress points which David Cameron and I raised after visiting Iraq last week - in particular the creation of an international group and the emphasis laid on a negotiated settlement within Iraq."