Shares in WPP sank on Wednesday after the firm cut its full-year revenue forecasts amid slowing advertising demand from consumer goods firms.
The advertising giant's stock plunged over 11% to 1,409p in midday trading after updating the market on its half-year performance.
WPP posted a 1.9% rise in revenue to £7.4 billion in the first six months of the year, but like-for-like net sales fell 0.5%.
The company, headed up by Sir Martin Sorrell, said it saw pressure on client spending in the second quarter, particularly in the fast-moving consumer goods sector.
As a result, WPP forecasts that full-year like-for-like revenue and net sales will come in between zero and 1% growth.
It had previously pencilled in 2% growth.
Current trading is also challenging, WPP added, with all regions - except the United Kingdom, Latin America and Central & Eastern Europe - showing lower revenue in July compared with the same month in 2016.
The company said "all sectors were down", with advertising and media investment management and data investment management the most affected.
WPP said: "Competition is fierce and as image in trade magazines, in particular, is crucial to many, account wins at any cost are paramount.
"There have been several examples recently of major groups being prepared to offer clients up-front discounts as an inducement to renew contracts.
"Our industry may be in danger of losing the plot."
Pre-tax profit rose more than 52% to £779 million in the period, but the tone of the trading update was unmistakably gloomy on the prospects for this year and next.
Graham Spooner, investment research analyst at The Share Centre, said that given WPP is a bellwether of the advertising industry, it is no surprise that its shares have tanked.
"WPP is very much seen as the bellwether of the advertising industry and as such is widely regarded as a global economic barometer and so it is unsurprising the shares have reacted in the way they have.
"It is likely that the market will continue to concentrate on the group's gloomy outlook for growth 2018, particularly as it expects little in the way of global GDP growth next year," he said.