In quotes: Coulson steps down
Figures from inside and outside Westminster have been giving their reaction to the resignation of the prime minister's communications chief, Andy Coulson.
Mr Coulson previously resigned from the News of the World in 2007 after a reporter was jailed for hacking into the voicemails of royal aides - although he said he was not aware that the practice was going on.
"I am obviously very sorry to see him go because he did a very good job as director of communications, both for the government and for the country.
"But I understand why he wants to go - the things that happened in the News of the World, the stories about that.
"Obviously, he resigned at the time and I've always felt he has been punished for the same offence twice, but I quite understand his decision and wish him well for the future."
Ed Miliband - Labour leader
"Labour's been saying for some months that there are real questions about Andy Coulson's ability to do his job given the cloud of allegations hanging over him.
"We think he should have gone earlier. He has now done the right thing.
"I think there are questions about David Cameron's judgement about hanging on to him as long as he did."
John Prescott - former deputy prime minister
"I think he should have gone before, but I think the real point is about Cameron's own judgement.
"I did write to him 20 months ago to say, 'Look, it's been announced in the Guardian that there are 3,000 people on this list [of alleged hacking targets].'
"The argument from Coulson has been that it was a rogue reporter. We now know by the many incidents and the re-opening of inquiries, that's not the case...
"[So] to hear Mr Cameron say he's still very relaxed... as I said in my letter... if you feel a relaxed attitude to those allegations, it seriously calls into question your judgement."
Lance Price - former Labour communications chief
"There are very important debates that Downing Street has to get involved in about media ethics... and it was very difficult for Downing Street to take any kind of moral high ground with the media while Andy Coulson was still there.
"I think he'd come to the conclusion that he was going to have to go and was looking for the right opportunity to do it. As we know, he is a backroom man, but it's still a very high-profile position and you can't do that when your credibility is being questioned.
"The other question of why now - we don't know what else is going to come out through the legal process... maybe he knows something we don't about something else that's going to come out into the public domain."
Nick Robinson - BBC political editor
"[David Cameron] has come to rely on Andy Coulson for a connection with the real world.
"Andy Coulson is an Essex boy and proud of it, a man not from the old Etonian background of David Cameron and many of his closest allies.
"It was particularly valuable to him to have someone in the room who said, 'That's not how it'll play in the real world, that's not how the red-top newspapers will see it, that's not what matters to most ordinary voters'."
Tom Watson - Labour MP
"This is the second job that Andy Coulson has resigned from for something he claims to know nothing about.
"His departure creates serious questions over the prime minister's judgement and points to the need for a deeper investigation into the affairs of Rupert Murdoch's News Corp.
"It's a mark of the man that he would sneak out a statement on a Friday morning on a busy news day.
"Spin and obfuscation is all we get from Downing Street - we need to get to the truth."
Kevin Maguire - the Daily Mirror
"He has been on the Downing Street death row because these allegations kept coming... but there will be questions too about David Cameron's judgement because he knew about the allegations when he took Andy Coulson into No 10.
"Andy Coulson was aware of those problems, but I think the lure of No 10, the chance to get in there, see how it works at the heart of power... [meant] he put his own concerns to one side and he moved across.
"But that is the problem. It's very different in politics when you are in opposition from when you are in government."
Sheila Gunn - former press secretary to John Major
"For David Cameron, he would obviously have come to trust - and probably become friends - with Andy Coulson, and in your early days, early months as prime minister, there's an awful lot to be said for having people like that around you to ease the transition.
"I think that when journalists were talking to Andy Coulson they could hear David Cameron, [he was] almost a mirror image, he would be reflecting the views and attitudes of the prime minister."
Max Clifford - public relations executive
"It's a surprise, but it's not a surprise. The story looked like it was going away at the end of last year, but it's back now and it's got legs.
"If Andy believed - based on all the information he has and all the information the prime minister has - that he was going to win this battle he wouldn't have resigned."
Steve Richards - the Independent
"It's one of the most difficult, weirdest jobs in politics, this projecting a case to the media 24 hours a day, and he was quite good at it.
"David Cameron's popularity within the media noticeably improved when he got that job, so I don't blame David Cameron for putting him in there."
John Whittingdale - culture and media select committee chairman
"The committee concluded that we did not believe it was just one rogue reporter. Clearly there was something wrong in the News of the World newsroom at that time.
"[But] I don't think Andy Coulson or David Cameron was to know that revelations were to come out much later that would re-open the whole thing."
News of the World
The newspaper said it would not be making any comment about the resignation.