Tesco has announced that chief executive Philip Clarke is to quit the group on the back of “challenging” trading conditions.

Clarke will be replaced by Dave Lewis, currently president of Unilever’s personal care business, on 1 October 2014.

The group said sales and trading profit in the first half of the year were below expectations, amid tougher-than-expected trading conditions.

Bryan Roberts, director of retail insights at Kantar Retail, said Tesco’s problems were both self-inflicted and a consequence of the changing grocery market.

“Some of Tesco’s problems are self-inflicted, such as the lack of hours and the unnecessary promotional complexity, but some are external and structural in nature, such as the rapid rise of the discounters and also the burgeoning presence of high street discount stores,” he said.

He praised Tesco’s progress in convenience store refreshes, but said it needed to “re-establish a sense of purpose in a business that seems to have lost its direction in a market that is changing around it”.

Tesco chairman Sir 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 with fresh perspectives and a new profile.”

Clarke added: “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.  I will do everything in my power to support him in taking the company forward through the next stage of its journey.”