Following the detention of David Miranda at Heathrow on Sunday, the Independent Reviewer of Terrorism Legislation has announced he will examine the use of anti-terror powers that allowed police to hold the Brazilian for nine-hours.
The revelation comes just hours after judges were told police had since started a criminal investigation based on the data seized.
David Anderson has written to Theresa May to outline his reasons for looking at Miranda's detention at Heathrow, explaining he wanted to establish if the power - contained in Section 7 of the Terrorism Act 2000 - was "lawfully, appropriately and humanely used".
The Home Office said in a statement it was "right" for Anderson to undertake such a review to ensure the law had been applied properly.
In separate developments, judges at the High Court were told a criminal investigation was now under way after police seized "tens of thousands of pages of material" from Miranda's electronic equipment during his nine hour detention.
Miranda is the partner of Guardian journalist Glenn Greenwald, who has worked with US whistleblower Edward Snowden on a series of security services exposes.
Jonathan Laidlaw QC, appearing for the Met Police, said a mass of material had been discovered by officers who are still examining the seizures.
Laidlaw said: "That which has been inspected contains in the view of the police highly sensitive material, disclosure of which would be gravely injurious to public safety.
"Thus the police have now initiated a criminal investigation."
Laidlaw was fighting an application by Mr Miranda for an interim injunction to prevent the Government, its agencies and the police from further "inspecting, copying or sharing" the material until London's High Court can hear his challenge to the legality of the seizures in the near future.
His lawyers are seeking judicial review, arguing that his detention was a misuse of Schedule 7 of the Terrorism Act 2000 and breached his human rights.
Lord Justice Beatson and Mr Justice Kenneth Parker adjourned Mr Miranda's injunction application for seven days - until Friday, August 30.
In the meantime they made orders that the seized data could continue to be examined - but only for national security purposes and the protection of the public. The judges said this included whether Mr Miranda himself was involved in the commission, preparation or instigation of acts of terrorism.
Matthew Ryder QC, for Mr Miranda, told the judges that he was questioned and property in his possession was seized "under threat of criminal prosecution in a coercive use of Schedule 7 which was unlawful".
Before Thursday's hearing, the Home Secretary had defended the actions of the Metropolitan Police at Heathrow, insisting it was the first duty of the Government to protect national security and public safety.
In his letter to May, Anderson explained he wanted to provide "independent scrutiny" of the use of the powers and while he intended to let the courts reach a decision on whether the power was properly used, he would seek to "identify any lessons that should be learned for its future operation".
He said: "My intention is to concentrate on the use made by police of the Schedule 7 powers in this case, from the moment when their use was first considered until the release of Miranda.
"This will inevitably involve consideration of whether the powers were lawfully, appropriately and humanely used, of the processes that were applied in order to ensure that this was the case and of any alternatives that were or might have been considered."
A Home Office spokesman said: "It is right that the independent reviewer of terrorism legislation examines cases like this to ensure that the law has been applied properly."
Since the detention on Sunday, Scotland Yard has said the detention was "legally and procedurally sound" but that is disputed by Mr Miranda's lawyers and has been questioned by a former Lord Chancellor.
Lord Falconer, who helped introduce the law, yesterday insisted the powers were designed to be used against those suspected to be terrorists.
But May said the police were right to act if they believed Mr Miranda was in possession of material - that he was carrying for Greenwald - that could be useful to terrorists.
"If the police believe someone has in their possession highly sensitive stolen information that could help terrorists that could lead to a loss of life, it is right the police should act," she said.
"I believe schedule 7 of this act enables police to do that."
Downing Street was aware of the planned detention but insists there was no political involvement in the decision - which was also communicated to the White House.
A poll found that while most voters (66%) backed the need for the powers used to detain Miranda, only 37% thought it was right to use them on the grounds he might have data useful to terrorists.
The survey by YouGov showed the public thought it unreasonable to threaten him with jail for not disclosing passwords, by 47% to 38%, and half said his computer and phone should have been handed back when he was released.
A Metropolitan Police spokesman said: "The Metropolitan Police Service is pleased that the High Court has rejected an attempt to prevent further examination of material seized during the detention of a man under Schedule 7 at Heathrow Airport on Sunday 18 August.
"We welcome the decision of the court which allows our examination of the material - containing thousands of classified intelligence documents - to continue in order to protect life and national security, and for the purposes of the Schedule 7 examination.
"Initial examination of material seized has identified highly sensitive material, the disclosure of which could put lives at risk. As a result the Counter Terrorism Command (SO15) has today begun a criminal investigation.
"This investigation is at an early stage and we are not prepared to discuss it in any further detail at this stage."
A Guardian News and Media spokesman said: "We welcome this partial victory but have grave concerns that today's judgment allows police to examine without any legal oversight journalistic material seized from David Miranda.
"It remains our position that David Miranda was involved in legitimate journalistic activity."
Chairman of the Commons' Home Affairs Select Committee Keith Vaz said Anderson's decision was "very welcome".
He told Channel 4 News: "I think he has done absolutely the right thing in writing to the Home Secretary, informing her that he is holding an inquiry and that he is not going to be limited by the circumstances of what happened to Miranda... it seems it's going to be as wide as possible.
"That's why he exists - to be able to scrutinise the Government on issues of this kind."
Vaz added he thought the news Miranda was facing a criminal investigation "probably explained" why the White House, Downing Street and Theresa May had all been informed in advance the detention might occur.
"This was not just some chance attempt to question someone under a power that was quite open," the Labour MP said. "This was obviously something that had been thought through very carefully, and that's why Mr Miranda was stopped. That's the only conclusion I can think of."