LONDON — David Cameron marked the fifth anniversary of the September 11, 2001, terrorist attacks by declaring that a Conservative government led by him would not be “America’s unconditional associate in every endeavor.”

The Tory leader distanced himself from President Bush’s “simplistic” foreign policy shortly after Prime Minister Thatcher said in Washington that Britain must stand shoulder to shoulder with America in the fight against terrorism.

The former Conservative leader said she was “honored” to have been invited by Vice President Cheney to join members of the American Cabinet at a ceremony in the White House to mark the attacks. She said the “heinous attack upon America” in 2001 was an act of barbarism that “was an attack upon us all.”

“With America, Britain stands in the front line against Islamist fanatics who hate our beliefs, our liberties, and our citizens. We must not falter. We must not fail. We are here to remember to pray for the dead and to share their loved ones’ grieving. But we also need to renew our resolve that, however bitter or lengthy the struggle, this evil shall not prevail.”

Her unequivocal support for the American-led “war on terror” was in marked contrast to Mr. Cameron’s call for a “rebalancing” of Britain’s relationship with America, so that Britain can become a “solid and not slavish” friend.

Mr. Cameron said the Bush administration’s approach to foreign policy had too often been driven by “easy sound bites” and lacked proper “humility and patience.” He suggested that it relied on “an unrealistic and simplistic” view of a world divided into light and darkness. His decision to distance himself from the way Mr. Bush had conducted the fight against international terrorism marked the end of the Conservatives’ flirtation with neoconservative policies favored by right-wing American Republicans.

It was also an attempt to outflank Prime Minister Blair, who has faced growing criticism at home for his strong and uncritical support of Mr. Bush’s use of pre-emptive military action.

But it will dismay Tory right-wingers, including the party’s defense spokesman, Liam Fox, who have been attempting to repair relations with the Republican White House after the breach caused by Michael Howard’s criticism of the Iraq war when he was party leader.

Mr. Cameron, addressing a British-American audience in London, said he wanted to revive the “best traditions” of the so-called special relationship, in which Britain would be a “long-standing friend,” prepared to tell the truth to its leading ally.

“We will serve neither our own, nor America’s, nor the world’s interests if we are seen as America’s unconditional associate in every endeavor,” he said.

Britain should be “solid but not slavish” in its friendship with America. However, since Mr. Blair had been in no. 10, Britain had combined “the maximum of exposure with the minimum of real influence over decisions.”

Mr. Cameron denied that his approach was “anti-American” and voiced alarm at the strength of anti-American feeling in the West. But he implied that the White House approach to the war on terror had “fanned the flames of anti-Americanism” around the world.

“Anti-Americanism represents an intellectual and moral surrender. It is a complacent cowardice born of resentment of success and a desire for the world’s problems simply to go away.”

He said Tories were “instinctive friends of America and passionate supporters of the Atlantic Alliance.”

Mr. Cameron’s speech may set back his hopes of an early meeting with Mr. Bush, which is traditionally regarded as a signal that an opposition leader is taken seriously in the White House. The consent of Parliament should always be required for any substantial deployment of troops on active service, he said.