There were claims that a Government report was changed to make Saddam Hussein seem more dangerous than he actually was.
As Mr Blair's press officer, Mr Campbell would have had a lot of input into what information was released to the public.
Mr Campbell's resignation is thought to be a big blow to the Prime Minister.
Spin
They have worked closely together since Mr Blair became leader of the Labour Party in 1994.
He worked on national newspapers before joining Mr Blair's staff.
It meant he knew how the media worked and some people accused him of "spinning" stories - that means releasing information which made the Government look good.
The Prime Minister has not yet appointed a new press officer.
Meanwhile, the Hutton Inquiry continues.
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