UK police is facing renewed ire in the phone-hacking controversy after it recently threatened to invoke the Official Secrets Act against the journalist who broke the story that murdered teenager Milliy Dowler's phone had been hacked.

The Guardian's story about 13-year-old Dowler's phone being hacked at the behest of the News of the World was the turning point in the controversy that triggered a series of events that led to the closure of the tabloid, resignations in News International and withdrawal of the BSkyB takeover bid by media baron Rupert Murdoch.

Faced with considerable criticism for using the Official Secrets Act, the police decided not to pursue its legal bid against The Guardian reporter, Amelia Hill, who broke the story.

A representative of the Metropolitan police will now appear before MPs tomorrow to explain in private why it threatened to invoke the act in an attempt to force the Guardian to hand over notes and reveal sources behind its phone-hacking coverage.

The hearing tomorrow is both official and secret, leading to condemnation by campaigners for media freedom, which threatens to dash the police's hopes that the furore over their alleged attempt to strike at media freedom will die away, the newspaper said.

John Kampfner, chief executive of campaign group Index on Censorship, said: "Holding this hearing behind closed doors would be a damaging move for parliament and the Metropolitan police. It is important that the police explain their actions openly".

"The attempt to use the Official Secrets Act on a journalist was an outrageous attack on free speech and those responsible should explain themselves not just to parliament but to the country," he added.

Keith Vaz, chair of the Home Affairs Select committee, said: "I have asked the Metropolitan police to give the committee a full explanation of why they took this action and to provide us with a timeline as to exactly who was consulted. It is essential that we get the full facts." 

Alan Rusbridger, editor-in-chief of the Guardian, said: "I just hope that, in our effort to clean up some of the worst practices, we don't completely overreact and try to clamp down on perfectly normal and applaudable reporting. This was a regrettable incident, but let's hope it's over."

The police application was formally made under the Police and Criminal Evidence Act, but with a claim that Hill had committed an offence under the Official Secrets Act by inciting an officer from Operation Weeting -the police investigation into phone hacking - to reveal information.