As the first bombs began falling on Iraq on March 20, 2003, Stephen Harper rose in Canada's Parliament to deliver some tough words on Saddam Hussein.

Mr Harper, now the Canadian Prime Minister, branded the Iraqi regime a “dangerous aggressor” and said that “possession of chemical, biological or nuclear weapons by terrorists would constitute a direct, undeniable and lethal threat to the world, including to Canada and its people”.

Tough as they were, the words were not his own. They belonged — improbably — to John Howard, then Prime Minister of Australia, who had used them in Parliament two days earlier.

Now the oratory has come back to bite Mr Harper as he campaigns for elections on October 14.

Opposition researchers unearthed both speeches and the Liberal Party has supplied side-by-side videos of Mr Harper and Mr Howard that make clear that almost half of his defining speech on Iraq was lifted from the Australian leader.

“How does a political leader in Canada's Parliament, on such a crucial issue, end up giving the exact same speech as another country's leader?” Bob Rae, the foreign affairs spokesman of the Liberals, said.

Mr Rae called the videotape further evidence “of how Canada's foreign policy is now in lockstep with the right-wing foreign policy of the Bush Administration”.

The scandal puts Mr Harper alongside Gordon Brown and Joe Biden in the ranks of alleged political plagiarists. Mr Brown was accused of lifting parts of his “sometimes people say I'm too serious” speech at the Labour conference in 2007 from Al Gore, the former US Vice-President.

Mr Biden, the Democratic vice-presidential candidate, suffered a similar embarrassment when he lifted parts of a speech from Neil Kinnock, the former leader of the Labour Party.

Aides thought so highly of the address by Mr Harper that they printed it out in pamphlet form and posted out thousands of copies.

Former aides to Mr Harper rejected any suggestion that both men were following a script prepared by the Bush Administration.

The Conservatives sought to limit the damage by announcing that the speechwriter would resign. Owen Lippert, an expert on intellectual property, said that neither Mr Harper nor his aides had known about the plagiarism.

“Pressed for time, I was overzealous in copying segments of another world leader's speech,” he confessed.

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