The attorney general, Lord Goldsmith, last night threatened newspaper editors with section five of the Official Secrets Act.
The exceptional move came after the Daily Mirror yesterday morning broke the story of the alleged Bush plan, referring to details in the memo without publishing it.
The document is a transcript of a conversation between Bush and Prime Minister Tony Blair at the White House on April 16 2004. It apparently shows that Blair dissuaded Bush from carrying out the plan.
Daily Mirror editor Richard Wallace said last night that no objection was made by Number 10 when it was notified before the story's publication.
"We made Number 10 fully aware of the intention to publish and were given 'no comment' officially or unofficially. Suddenly, 24 hours later we are threatened under section five [of the Official Secrets Act]," he said.
Section five of the Act makes it an offence to have come into the possession of government information, or a document from a crown servant, if that person discloses it without lawful authority. The prosecution has to prove the disclosure is damaging.
Two civil servants have been charged with offences relating to the leaking of the memo.
The White House has called the Daily Mirror story "outlandish", while Al Jazeera said it is investigating the report.
"If the report is correct then this would be shocking and worrisome not only to Al Jazeera but to media organisations across the world," the network said in a statement.
Al Jazeera's Arabic news channel has angered the US and UK governments with its coverage. During the 2003 invasion, of Iraq it broadcasted images of dead and captured coalition troops, and it has also shown footages of terrorist hostages and broadcast messages from Osama bin Laden.
Al Jazeera is to launch an English-language channel in the West in the spring, and has hired respected broadcaster Sir David Frost as part of efforts to win over its critics.
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