Attorney General Lord Goldsmith has said the decision to drop the trial of GCHQ whistle-blower Katharine Gun was taken for legal, not political reasons.
Katharine Gun said the e-mail asked for help bugging key states
Mrs Gun was accused of leaking US spies' request for UK help to bug UN delegates ahead of the Iraq invasion.
Senior lawyers felt they could not disprove her defence that she was acting to prevent an unlawful war.
Downing Street says the Official Secrets Act is now being reviewed in the wake of the case's collapse.
Tony Blair's official spokesman said the review was "to see whether, and I stress whether, any changes need to be made".
He argued it was a "matter of common sense" to review the act after such a case.
The prime minister denies intervening in the case and says the decision had nothing to do with advice on the legality of war.
Lord Goldsmith said they could prove the Official Secrets Act was breached.
But senior government lawyers did not believe it was easy to overcome Mrs Gun's defence of "necessity" - that she felt a duty to act to prevent an unlawful war.
There has been widespread speculation the trial was avoided because Mrs Gun's defence had requested full details of Lord Goldsmith's advice to the government over the legality of the war.
The government has steadfastly refused to publish all of the advice.
At his monthly media conference, Tony Blair said: "It isn't to do with some issue to do with the publication of his legal advice."
He said the foreign secretary was contacted because he was responsible for Government Communications Headquarters (GCHQ), the intelligences services' listening station.
But he refused to be drawn on allegations that Lord Goldsmith had advised against the legality of war up until January 2003 when he changed his mind.
Later, Solicitor General Harriet Harman said the defence of "necessity" involved many issues.
"It was not an issue about the attorney-general's advice about the legality of the war, it was not an issue about second-guessing the extent to which the jury trusted the government," she told MPs.
Nor was it true that the case had been dropped because the government did not want certain evidence shown in court, she added.
Mrs Gun's legal team served documents on the government on Tuesday demanding to see any advice given to ministers about the legality of the war.
But a government spokesman insisted on Wednesday the decision to drop the case was taken before the demand for documents was made.
Tony Blair is expected to face tough questions at Number Ten
Mrs Gun, who was sacked from GCHQ in June and charged on 13 November for leaking the e-mail to the Observer newspaper, said: "Obviously I'm not prone to leak secrets left, right and centre... but this needed to get out, the public deserved to know what was going on at the time.
"I was pretty horrified and I felt that the British intelligence services were being asked to do something that would undermine the whole UN democratic processes."
The memo, from January last year, reportedly said the National Security Agency had begun a "surge" in eavesdropping on UN Security Council countries crucial to the vote on a second resolution for action in Iraq.
Officials from Angola, Cameroon, Chile, Bulgaria, Guinea and Pakistan all had their phones tapped in what a newspaper described as a "dirty tricks" operation.