David Cameron said today that he was "extremely sorry" for the furore his appointment of Andy Coulson as his communications chief had caused.
In a Commons statement, the Prime Minister said that with the benefit of "20:20 hindsight" he would not have given the former News of the World editor the job.
Mr Cameron told MPs that he would owe a "profound apology" if it turned out that the assurances Mr Coulson had given him that he was not involved in phone-hacking had turned out to be false.
Mr Cameron said that if Mr Coulson had lied about phone hacking at his time at the News of the World then he should face "severe" criminal charges.
He added: "If it turns out I have been lied to that would be a moment for a profound apology, and in that event I can tell you I will not fall short."
He acknowledged that if he had known when he appointed Mr Coulson what he knew now, he would never have offered him the job.
"Of course I regret, and I am extremely sorry about the furore it has caused. With 20:20 hindsight and all that has followed I would not have offered him the job and I expect that he wouldn't have taken it," he said.
"But you don't make decisions in hindsight, you make them in the present. You live and you learn and believe you me, I have learned."
Mr Cameron also strongly rejected suggestions that his chief of staff Ed Llewellyn had been wrong to prevent him receiving a private briefing on the police investigation into phone hacking by former assistant commissioner John Yates.
He said Mr Llewellyn's response to Mr Yates had been had cleared in advance by his permanent secretary, Jeremy Heywood.
"Just imagine if they had done the opposite and asked for, or acquiesced in receiving, privileged information - even if there was no intention to use it," he said.
"There would have been quite justified outrage. To risk any perception that No 10 was seeking to influence a sensitive police investigation in any way would have been completely wrong."
Mr Cameron also dismissed claims that he had broken the ministerial code of conduct by meeting News International executives - including Rebekah Brooks - while Rupert Murdoch was bidding to take over BSkyB.
"The Cabinet Secretary has ruled very clearly that the code was not broken - not least because I had asked to be entirely excluded from the decision," he said.
He said that politicians now needed to act decisively to deal with the issues raised by the phone-hacking scandal.
"There is the history of missed warnings - select committee reports, information commissioner reports - missed by the last government but, yes, also missed by the official opposition too," he said.
"What the public expects is not petty point-scoring, but what they want, what they deserve, is concerted action to rise to the level of events and pledge to work together to sort this issue once and for all."
Mr Cameron said: "What the public expects is not petty point scoring, but what they want, what they deserve, is concerted action to rise to the level of events and pledge to work together to sort this issue once and for all."
But Labour leader Ed Miliband dismissed Mr Cameron's words on Mr Coulson as "not enough", and claimed there had been at least five occasions where his office ignored damning information about the former News of the World editor.
"The country has a right to expect that the Prime Minister would have made very effort to uncover the information about Andy Coulson to protect himself and his office," Mr Miliband said.
"Yet the pattern of events suggests the opposite: that the Prime Minister and those around him made every effort not to hear the facts about Andy Coulson."
Mr Miliband said Metropolitan Police Assistant Commissioner John Yates had offered to brief the premier on the phone-hacking situation last September, but Number 10 chief of staff Ed Llewellyn had rejected the suggestion.
"The question is why," he added. "Because the Prime Minister was compromised by his relationship with Mr Coulson and therefore could not be told anything at all about an investigation concerning a member of his own staff. He was hamstrung by a conflict of interest."
The Labour leader went on: "This cannot be put down to gross incompetence. It was a deliberate attempt to hide from the facts about Mr Coulson.
"The Prime Minister was caught in a tragic conflict of loyalty between the standards of integrity that people should expect of him and his staff and his personal allegiance to Mr Coulson. He made the wrong choice."
Mr Miliband suggested that Mr Cameron's conflict of interest led to the resignation of Met Commissioner Sir Paul Stephenson over the weekend after it emerged that the force had hired Mr Coulson's former News of the World deputy Neil Wallis as a media adviser.
"Sir Paul Stephenson was trapped," Mr Miliband said. "He was trapped between a Home Secretary angry about not being told about the hiring of Mr Wallis and Sir Paul's belief in his own words that doing so would have compromised the Prime Minister."
Mr Miliband added: "It's not about hindsight. It is not about whether Mr Coulson lied to him. It is about all the information that the Prime Minister ignored."
But Mr Cameron shot back: "Stop punting feeble conspiracy theories and start rising to the level of events."
He insisted he was being completely transparent by publishing details of all his meetings with senior media figures since the general election, and criticised his counterpart for only releasing equivalent material since becoming Labour leader last September.
The Premier also pointed out that Rupert Murdoch claimed yesterday that the politician he was closest to was Gordon Brown as chancellor.
In his statement, Mr Cameron said he was widening the remit of the judicial inquiry set up under Lord Justice Leveson.
He said it would now look at broadcasters and social media, as well as the press, if there was any evidence they had been involved in criminal activities.
It will consider the individual conduct of press, police and politicians as well as the relationship between them.
It will also look at all relevant police forces and not just the Metropolitan Police.
Mr Cameron said the inquiry would begin work immediately and would deliver its first report within 12 months.
The Prime Minister said he believed that the current crisis should also be a moment to take a look at the broader culture of policing.
"I want to see radical proposals for how we can open up our police force and bring in fresh leadership," he said.