LONDON (AP) — Shares in WPP, the world's biggest advertising company, have nosedived after it warned about weaker earnings.

The London-based company, which has struggled amid a shift of advertising to internet platforms and social media, said Thursday it saw a slowdown in the third quarter, leading to "a more cautious outlook for the rest of the year."

Third quarter revenue was down 0.8 percent to 3.76 billion pounds ($4.9 billion), partly due to a slowdown in business in North America. CEO Mark Read, who took over from founder Martin Sorrell in September, said the company has been "too slow to adapt" to structural change in the industry.

Shares in WPP on the London Stock Exchange were down 18 percent, at 867.40 pence, in midday trading.