An initial examination of material seized from David Miranda at Heathrow Airport "has identified highly sensitive material, the disclosure of which could put lives at risk," Scotland Yard said.
"This [criminal] investigation is at an early stage and we are not prepared to discuss it in any further detail at this stage," a spokesperson added.
The independent reviewer of terrorism legislation said he will examine the detention of David Miranda at Heathrow Airport.
Nick Clegg has backed David Cameron's decision to send a top civil servant to The Guardian to urge them to destroy classified data.
The Guardian journalist whose partner was held at Heathrow Airport hits out at the UK as he gives ITV News his first British TV interview.