Commenting on the development, Tesco chairman Richard Broadbent said: "Having guided Tesco through a substantial re-positioning in challenging markets, Philip Clarke agreed with the Board that this is the appropriate moment to hand over to a new leader."
"We are pleased to announce today the appointment of Dave Lewis, President, Personal Care at Unilever, as our Chief Executive from 1 October, 2014," Broadbent added.
Giving outlook for the first half of the current year, Tesco said: "Current trading conditions are more challenging than we anticipated at the time of our first quarter interim management statement on 4th June."
"The overall market is weaker... this means that sales and trading profit in the first half of the year are somewhat below expectations," it added.
Tesco said Lewis, currently working as Unilever's Global President, Personal Care, has been responsible for a number of business turnarounds as well as leadership in areas of significant strategic priority.
The company said Philip Clarke will continue as CEO until October 1 and will remain available to support the transition until the end of January 2015.