Retail giant Tesco has announced the resignation of its boss Philip Clarke as it unveiled a profit warning after a "challenging" second quarter.Clarke, who was appointed Chief Executive Officer (CEO) in 2011 and has been on the board since 1998, "agreed [...] that this is the appropriate moment to hand over to a new leader with fresh perspectives and a new profile", the company said on Monday.Tesco said that Dave Lewis, a director at BSkyB who is also the President of Unilever's Personal Care division, will succeed Clarke as CEO in October. The grocery giant said current trading conditions "are more challenging than we anticipated" at the time of its first-quarter update in early June."The overall market is weaker and, combined with the increasing investments we are making to improve the customer offer and to build long term loyalty, this means that sales and trading profit in the first half of the year are somewhat below expectations."Clarke said he would stay on until January 2015 to support Lewis and will remain Chairman of Tesco's joint venture with CRE in China until that date.Clarke said: "Having taken the business through the huge challenges of the last few years, I think this is the right moment to hand over responsibility and I am delighted that Dave Lewis has agreed to join us. Dave has worked with Tesco directly or indirectly over many years and is well-known within the business."Lewis will receive a basic salary of £1.25m along with "standard benefits commensurate with his position". He will also pocket £525,000 and stocks in lieu of his current year cash bonus and deferred share awards from Unilever.BC