FILE PHOTO: Martin Sorrell, chairman and chief executive officer of WPP, the world's largest advertising company, speaks at the Confederation of British Industry's (CBI) annual conference in London, Britain November 21, 2016. REUTERS/Stefan Wermuth/File Photo

LONDON (Reuters) - Shares in WPP fell 4 percent at the open on Monday after Chief Executive and founder Martin Sorrell quit, leaving the group without a boss at a time of huge change in the industry.

Shares in the group, which are already down 30 percent this year, were down 4 percent at 0703 GMT. Analysts and peers have speculated that the group of 200,000 people could be broken up without Sorrell at the helm.