A date has not yet been revealed for the departure of prime minister Tony Blair's chief spin doctor and top confidant, nor has a successor been revealed.
In a resignation statement, Campbell said he had no intention of taking 'another big job as such at this stage' but instead planned to write, broadcast and make speeches.
He also said he wanted to 'get more involved in grassroots sports development'.
Blair described Campbell, who he has worked alongside since 1994, as 'an immensely able, fearless, loyal servant of the cause he believes in,' adding 'I wish him every success - he was, is, and will remain a good friend'.
Campbell paid tribute to Blair describing him as 'someone I believe history will judge as a great transforming leader'.
Speculation that Campbell was planning his departure has been building over the past months and his decision to reveal his plan to resign comes just one day after Blair appeared in front of the Hutton Inquiry into the death of scientist Dr David Kelly.
Campbell said: 'I had intended to leave last summer, but as the Iraq issue developed, the Prime Minister asked me to stay on to oversee Government communications on Iraq'.
Campbell said he and Blair had decided on 7 April that he would 'definitely leave this summer' and that he would be 'available in the next few weeks to assist the handover to my successor.'
Campbell said his replacement would be announced 'shortly'.
As to his plans to write, Campbell said he was 'keen to write not just about politics but other areas of our national life, such as sport.'
If he did so Campbell would follow the example of Charlie Whelan, formerly spin doctor to chancellor Gordon Brown, who upon leaving government wrote a sports column for London's Evening Standard and the Observer.
Former Labour Party comms director David Hill is one of the front-runners for the post.
Labour Party chair Ian McCartney said: 'I was delighted when [Campbell] gave me today his absolute commitment to carry on campaigning and working for the party. I'm going to hold him to it'.