Downing Street media chief Alastair Campbell is to step down, it has been announced.
Campbell said it was a "great privilege" to serve Tony Blair
Mr Campbell, Tony Blair's director of communications and strategy, said his family had paid a heavy price for the "real and intense" pressures of his job and it was "time to move on and do other things".
He will be succeeded by David Hill, Labour's communications chief in the 1997 general election and former press aide to ex-Labour deputy leader Roy Hattersley.
A date for Mr Campbell's departure has not been set, though he said he would leave Downing Street - along with partner Fiona Millar, who works for Cherie Blair - in "a few weeks".
Downing Street said Mr Hill would operate within a new
communications structure, details of which will be announced next week.
Mr Campbell said his resignation was not related to the Hutton inquiry into the death of government weapons expert Dr David Kelly, saying he had agreed in April with Mr Blair that he would leave his post this year.
He told the BBC he wanted "to get a life back for me and my family".
But it had been "an enormous privilege to work so closely in opposition and in government for someone I believe history will judge as a great
transforming prime minister", he said.
Mr Blair said Mr Campbell had been "an immensely able, fearless, loyal servant of
the cause he believes in".
He added: "He was, is, and will remain a good friend."
Mr Campbell, a father-of-three, returned the sentiment, saying that he had a friendship with Mr Blair which would endure .
He said: "He gave me a big job, a big challenge and I strove at all times to do it to the best of my ability, and hopefully made a difference for the better."
He said he now planned to write, broadcast and make speeches, but did not want to take on "another big job".
Asked by BBC political editor Andrew Marr whether his plans included publishing the diaries he has kept while working for Mr Blair, he said: "Any books that I write will be some time off."
On his reputation as a tough political operator, he said he didn't consider himself "ruthless", but added: "I have always been driven. I have always believed in what I believe very deeply."
Mr Campbell said he had planned to leave his job last summer but stayed on after being asked by the prime minister to oversee government communications on Iraq.
The former newspaper journalist said his resignation was announced on Friday because he felt it would be wrong to do so while the Hutton inquiry was sitting.
He said he would continue to assist the investigation if required.
Mr Blair said: "The picture of Alastair Campbell
painted by parts of the media has always been a caricature.
"The Alastair Campbell I know is an immensely able, fearless, loyal servant
of the cause he believes in, who was dedicated not only to that cause but to his
"He is a strong character who can make enemies but those who know him best,
like him best."
Mr Campbell said there was "no better job than the one I have been
He said he looked back "with pride and satisfaction" at his role in helping Labour to two election victories and assisting ministers in making "historic changes" while in government.
Mr Campbell started working with Mr Blair after he became Labour leader in 1994 and became the prime minister's official spokesman after Labour's election victory in 1997.
He moved to his role as communications director in 2001.
He said there were "huge upsides" to his job, including "the knowledge that you are witnessing history in the making".
But he added: "There are downsides too and these are mostly borne by your family.
"The reality is that in some jobs, and this is one of them, there is no such
thing as a day off, or a night off, or a holiday without interruption.
"The pressures are real and intense, but in doing the job you learn to live
with them. It is your family that pays a price."
He said Ms Millar plans to return to freelance journalism.
"We have three wonderful children and we look forward to spending a lot more
time with them," he added.
The Burnley football fan said he planned to write not only about politics, but also about sport, and get involved in grassroots sports development.
He said he also plans to devote more time and effort to the Leukaemia Research Fund, for which he ran the London Marathon earlier this year.
But he said politics was "a passion of my life" and would remain a close interest, saying: "I will continue to help the political causes I believe in in any way I