Hugh Grant said the fight against phone hacking is a "job half done" and criticised some members of the culture committee in a press conference at the Liberal Democrats annual autumn party conference.

The actor, who has been named as a "core participant" in the inquiry into phone hacking at the News of the World and other media, said that Scotland Yard's attempt to force The Guardian to disclose the source who told it Milly Dowler's phone had been hacked was "deeply mysterious".

"It's a very worrying, upsetting development," Grant said at a packed press conference, as LibDem children's minister Sarah Teather addressed the main hall elsewhere at conference . "We had come to the view that the new inquiry… were good cops. So for them to turn on their fellow goodies in this battle is worrying and deeply mysterious."

Appearing with Martin Moore, one of the founders of the Hacked Off campaign, and Dr Evan Harris, a former LibDem MP, Grant said that he would be attending each of the party conferences in order to keep the pressure on politicians.

"They really had no choice back in July, the revelations were so shocking that they had to talk a good game," he said, ahead of a fringe conference event with Index On Censorship. "Whether or not they will play a good game remains to be seen."

Criticising some members of the Culture, Media And Sport Committee, who questioned Rupert and James Murdoch in July following the phone hacking revelations, Grant said that he was "just slightly disappointed in them" and said they had appeared "starstruck" by the Murdochs.

Grant also said that he suspected broadcast media were included in the terms of the Leveson inquiry because of a "Tory plot" to attack the BBC.

The actor added that just because he was famous did not mean that he had given up his right to privacy. "Publicity is not what you make your living with," he said.

Grant also said that despite visiting the party conferences and becoming something of a public face against phone hacking, he had no political ambitions of his own.

"I'm quite enjoying it but I have no Ronald Reagan plans," he said. "I don't have the brainpower."

Martin Moore said that regarding the Metropolitan Police's attempt to force The Guardian to disclose its sources, it was now down to politicians to act.

"We think it's very worrying, we think the use of the official secrets act particularly seems rather astonishing and inexplicable," he said. "We need to ear from the police why they acted in this way… We need to hear from politicians (who so far) have not stepped forward."

Dr Evan Harris said that the LibDems would be debating phone hacking on Monday.

"Ultimately it's going to be politicians that get the job done, to get this thing fixed," Harris said.