The Met, also known as Scotland Yard, is seeking a court order to force The Guardian to disclose its sources under the Official Secrets Act, usually used to protect sensitive information related to national security or espionage. The Met claims that the paper may have violated the Act in July when reporters Amelia Hill and Nick Davies revealed that murder victim Milly Dowler's phone had been hacked.
"Operation Weeting (the phone-hacking inquiry) is one of the MPS's most high profile and sensitive investigations so of course we should take concerns of leaks seriously to ensure that public interest is protected by ensuring there is no further potential compromise," the police said in a statement.
Alan Rusbridger, the editor of The Guardian, said on Friday: "We shall resist this extraordinary demand to the utmost."
Labour MP Tom Watson also condemned the Met's motion: "It is an outrageous abuse and completely unacceptable that, having failed to investigate serious wrongdoing at the News of the World for more than a decade, the police should now be trying to move against the Guardian. It was the Guardian who first exposed this scandal."
Scotland Yard was accused of botching the initial investigation into News Of The World phone hackings when they first came to light in 2006. Only reporter Clive Goodman and P.I. Glenn Mulcaire were convicted of phone hacking then, but the Dowler revelations suggested that the practice had been more widespread.
Sir Paul Stephenson, chief of the Metropolitan Police Force, and John Yates, assistant commissioner of Scotland Yard, resigned in the wake of the revelations in July.
Read The Guardian's full report here.