One of the most senior Republicans to back the Iraq surge plan said today the strategy was failing, a surprise move that signalled how close Mr Bush is to losing a significant bloc of his own party on the war.

Richard Lugar, the top Republican and former chairman of the Senate Foreign Relations Committee, called for an immediate drawdown of US troops, adding that any delay would severely limit US options in Iraq.

As a moderate but respected voice on foreign policy, Mr Lugar’s break with the White House could provide political cover for more Republicans wanting to desert Mr Bush over the war.

Mr Lugar’s move was all the more surprising because it came three months before General David Petraeus, the Iraq ground commander, delivers his potentially pivotal progress report to Congress on September 15. Most Republicans who have reservations about the war have said they would wait until then before they decided whether to defect.

Republican support in the Senate has been crucial to Mr Bush’s success in staving off Democrat-inspired anti-war legislation. Although Mr Bush is still some way off losing his veto-proof majority in the chamber, Mr Lugar’s defection adds an influential voice of Republican opposition at a time when support for his Iraq policy is fast eroding within the party on Capitol Hill.

“This is a terrible blow to the administration,” said Ray LaHood, a Republican congressman from Illinois who has said he will wait for General Petraeus’s report before deciding whether to withdraw his support for Mr Bush. “It will get a lot of Republicans thinking about the way forward. Dick Lugar is one of the most respected members of Congress. He is very thoughtful. He analyses things. He’s not a knee-jerk guy.”

In a hastily convened speech on the Senate floor, Mr Lugar said the Iraq troop surge was strengthening America’s enemies, harming its long-term national security interests, exhausting the military and increasing the likelihood of a chaotic retreat from Iraq.

Failure in Iraq is not inevitable, Mr Lugar said, but success requires Mr Bush to diminish the role of the US military and “place much more emphasis on diplomatic and economic options.”

He added: “Persisting indefinitely with the surge strategy will delay policy adjustments that have a better chance of protecting our vital interests over the long term. Our course in Iraq has lost contact with our vital national security interests in the Middle East and beyond.” With all eyes on the September progress report, General Petraeus and the White House have been trying to downplay its significance in recent weeks, amid expections that it will fail to placate war critics and doubters. The White House has also commissioned other reports from the intelligence services to try and dilute its findings.

In essence, General Petraeus is expected to argue for as much time as possible to allow the surge to succeed. But the reality for Mr Bush is that back in the US, Republican patience on Capitol Hill is wearing thin.

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