More newspapers became embroiled in the phone-hacking scandal on Thursday as the deputy prime minister said the crisis was a chance to clean up "murky" ties between politicians, police and the media.

After hacking allegations at Rupert Murdoch's News of the World closed the Sunday tabloid, it emerged that police have asked for files from a regulator which exposed the use of private investigators by other rival papers.

The scandal has also engulfed Prime Minister David Cameron, who faces questions over discussions he had on Murdoch's failed bid for pay-TV giant BSkyB, and his employment of former Murdoch editor Andy Coulson as his media chief.

Deputy Prime Minister Nick Clegg said a forthcoming judge-led public inquiry into the scandal, now widened to include broadcasters, should take strong measures.

"We have a once in a generation opportunity to really clean up the murky practices and dodgy relationships taken root at the very heart of the British establishment between the press, politicians and the police," he said.

But the Liberal Democrat leader said Cameron had been "very categorical that no inappropriate discussions took place" with Murdoch's aides over the BSkyB bid, which collapsed earlier this month.

Opponents have seized on Cameron's comments as an admission that he did have conversations of some kind with Murdoch's lieutentants over the deal for full control of Britain's biggest satellite broadcaster.

Public outrage has until now focused on Murdoch. with public revulsion at claims that the News of the World hacked the voicemails of murdered teenager Milly Dowler.

The ageing tycoon was hit with a foam pie during a parliamentary hearing on Tuesday.

The crisis has also prompted Australia to move to introduce a legal right to privacy and in the United States spawned an FBI probe into Australian-born Murdoch's US-based News Corp. empire.

But other media groups are now under the spotlight after Britain's Information Commissioner's Office said police had requested files from a 2006 inquiry into the use of private investigators by newspapers.

That inquiry found that the Daily Mail had made 952 requests to private detectives for confidential details; The People made 802 requests, the Daily Mirror 681 and The Mail on Sunday 228. The News of the World was in fifth place with 228.

"The information was handed over to the police three months ago. It was at their request," a spokesman for the Information Commissioner's Office told AFP.

Scotland Yard said it would not discuss specific lines of inquiry.

The BBC quoted the Daily Mail as saying the information obtained may have been used for reasons of public interest.

The Trinity Mirror group, which owns the Mirror and The People, told the BBC its journalists worked within the law.

The scandal has been threatening to move onto other papers for some time.

On Tuesday an MP at the hearing which quizzed Murdoch claimed former Daily Mirror editor Piers Morgan had admitted to phone-hacking. Morgan, now a CNN presenter, has strongly rejected the allegation and demanded an apology.

Actor Jude Law is meanwhile suing The Sun, Murdoch's top-selling British daily, for allegedly hacking his phone. The Sun denies it.

The News of the World's former royal editor and a private investigator were jailed in 2007 for phone hacking but despite mounting evidence the practice was widespread, London police did not reopen their investigation until January.

Police have since arrested, questioned and then released 10 people over the affair.

They include Coulson, a former News of the World editor who quit in 2007 and months later become Cameron's communications chief. Coulson resigned from Downing Street in January.

Cameron told an emergency parliamentary session on Wednesday that "with 20-20 hindsight" he would not have hired Coulson.

A former deputy to Coulson at the News of the World, Neil Wallis, was previously the editor of The People, one of the papers named in the report on the use of investigators.

Wallis was arrested earlier this month but in another twist it emerged that he had been employed by the police in 2009-10 -- a revelation which forced Scotland Yard chief John Stephenson and top police officer John Yates to resign.