Mr Campbell, who was Tony Blair's communications director, said he had often wondered how stories about Cherie Blair were obtained.
He said: "I have never understood how the Mirror learned of Cherie's pregnancy. At the time only a tiny number of people in Downing Street knew she was pregnant."
Mr Campbell, himself former Mirror journalist, said he previously suspected stories were being leaked by Cherie's 'lifestyle consultant' Carole Caplin, who he believed was tipping off newspapers.
He said he had "since apologised" to Ms Caplin, after realising he was "completely wrong".
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