Ms Jane Kennedy is to leave the Northern Ireland Office in the cabinet reshuffle caused by the resignation of Immigrations Minister Ms Beverley Hughes.
Ms Hughes quit amid claims that she knew about an alleged visa scam, and was replaced by the Works and Pensions Minister, Mr Des Browne.
Ms Kennedy was appointed in June 2001 as the Northern Ireland Office security, policing and prisons minister.
The Northern Secretary, Mr Paul Murphy, said Ms Kennedy had performed exceptionally.
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"I will be sorry to lose her valued input and positive approach to the work of the Northern Ireland Office, and I wish her well in her new role and future career."