Phillis today publicly revealed that he was diagnosed with an early stage bone marrow cancer last summer and his treatment, though proceeding well, will involve a bone marrow transplant later in the year.
The search for his successor will be led by GMG chairman Paul Myners and Scott Trust chairman Dame Liz Forgan.
Myners said: "The board is hugely appreciative of everything that Bob has achieved in the eight years since he joined the group. GMG is stronger now than at any time in its history and our overriding purpose of guaranteeing the financial independence of The Guardian has been secured under his leadership."
Phillis has accepted an invitation to remain on the GMG board as a non-executive director, but will resign from the Scott Trust when he retires.
Phillis joined GMG in December 1997 with extensive experience of the television sector. He previously spent five years as deputy director-general of the BBC and his former positions include ITN chief executive and Carlton Communications group managing director.
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