Tesco said Clarke would continue as CEO until October 1 when he would step down from the board but will continue to be available to support the transition until the end of January 2015.
Tesco said current trading conditions were more challenging than it anticipated at the time of its first quarter interim management statement on June 4, and that sales and trading profit in the first half of the year were below expectations.
"Philip Clarke agreed with the board that this is the appropriate moment to hand over to a new leader with fresh perspectives and a new profile," said Chairman Richard Broadbent, who last month at Tesco's annual shareholders' meeting had endorsed Clarke as CEO.
Lewis is currently president of personal care at Unilever. Clarke was two-years into a multi-billion pound plan for Tesco's core British business, which accounts for two thirds of the group's sales and profits.
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He had invested in store refits, staff, product ranges and online services, had cut prices and dropped an industry leading profit margin target. But he had failed to deliver a durable increase in sales.
Tesco said the outlook for the full year would be influenced by the extent to which benefits from the investments it is making begin to be seen, by conditions in the overall market and by any steps that may be taken during the remainder of the year to improve its customer offer further.
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