Tony Blair is set to face questions on the dramatic collapse of the trial of GCHQ whistle-blower Katharine Gun.
Tony Blair is expected to face tough questions at Number Ten
She had been accused of leaking a secret e-mail from US spies allegedly requesting British help in bugging UN delegates ahead of the Iraq invasion.
The prime minister's monthly media briefing on Thursday is expected to be dominated by the dropping of the case against the ex-intelligence officer.
The government has denied claims the move was politically motivated.
There has been speculation ministers were worried about the disclosure of secret documents during the trial, particularly the advice from Attorney General Lord Goldsmith about the legality of war.
BBC political correspondent Paul Wilenius quoted sources at Westminster as saying it was Lord Goldsmith who ordered prosecutors to drop the proceedings against Ms Gun, who walked free on Wednesday when the prosecution offered no evidence.
Human rights group Liberty, which supported Ms Gun throughout her trial, said it was possible political intervention triggered the collapse of the case against her.
Liberty spokeswoman Shami Chakrabarti said: "One wonders whether disclosure in this criminal trial might have been a little too embarrassing."
The Liberal Democrats said the case had raised the possibility the attorney general's advice would have been published "at last" and called it a government "retreat".
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 BBC political correspondent Guto Harri said a government spokesperson insisted the decision to drop the case was taken before the demand for documents was made.
The same spokesperson suggested the case might have been dropped as Mrs Gun planned to argue she leaked the e-mail to save lives from being lost in a war.
That could persuade a jury, and would lead to the reputation of the Official Secrets Act - under which she was charged - being damaged.
Katharine Gun said the e-mail asked for help bugging key states
Harri said this suggested the government had made a political calculation that a random selection of a dozen jurors would be likely to be so instinctively anti-war than an acquittal would be likely.
Ms Gun, who was sacked from GCHQ in June and charged on 13 November for leaking the e-mail to the Observer newspaper, told a news conference on Wednesday: "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 the Observer described as a "dirty tricks" operation.
But Professor Anthony Glees of the Brunel Centre for Intelligence Studies was critical of Ms Gun's actions.
He told BBC Two's Newsnight programme that intelligence officers would be "hopping mad" about the case collapsing as it seemed "water-tight".
"This lady was doing what she absolutely ought not to have done and if she is allowed to get away with it then that is an extremely serious position for British security," he said.