Mark Read will step down as CEO of global advertising giant WPP after seven years in which fierce competition, technological change and economic challenges in key sectors and geographies led its share price to halve.

Read, who spent more than 20 years at the owner of Ogilvy and GroupM before he replaced founder Martin Sorrell in the top job, said he had built a simpler, stronger business by merging its major agencies to provide a more joined up service to clients.

But WPP, which lost its crown as the biggest ad group to France's Publicis last year, has struggled, losing some contracts to rivals and having to navigate the upheaval sparked by artificial intelligence, which gives clients the tools to create their own material.

The group said in February its growth would be flat at best this year and down 2% at worst.

Its shares have more than halved since Read took over in September 2018, and they fell to a five-year low in April.

They fell 1% in early trading on Monday.

WPP appointed Philip Jansen to chair its board at the beginning of the year, bringing across a corporate heavyweight who had previously led BT and Worldpay, and leading to speculation that he may look to shake up the group.

"Mark has played a central role in transforming the company into a world leader in modern marketing services, with deep AI, data and technology capabilities, global presence and unrivaled creative talent, setting WPP up well for longer-term success," Jansen said.

Read said it was the right time to hand over to a new leader.

"Our clients today rate us more highly than ever before, we now work with four of the world's five most valuable companies, and our revenues with our biggest clients have grown consistently," Read said.

Read will leave a simpler WPP after years of acquisitions by Sorrell, who turned WPP into the world's largest ad group through buying agencies including J. Walter Thompson in 1987, Ogilvy in 1989, Young & Rubicam in 2000 and Grey in 2004.