Annual revenues are now forecast to grow between only zero and 1%.
"For the short-term, we have to weather the storm," Sir Martin Sorrell, the chief executive, said in a statement.
Global billings dropped 4.7% and net sales fell 0.5% on a like-for-like basis in the first half of the year compared to 2016.
In a worrying development, conditions worsened in July as net sales dived 2.6%.
"All sectors were down, with advertising and media investment management and data investment management the most affected," the company said.
WPP had already revised its second quarter forecast and July was "behind budget and the quarter two revised forecast".
Investors took fright, even though the impact of last year’s loss of two big media accounts, AT&T and Volkswagen, had been well flagged all year.
WPP said revenues were not badly affected by June’s cyber attack.
The shares fell 159p to £14.31 and have dropped by a quarter since hitting £19.21 in March.
WPP’s rivals, Interpublic and Dentsu Aegis Network, have already warned at their half-year results that growth was slower than expected.
Ian Whittaker, analyst at investment bank Liberum, said WPP’s performance was "likely to disappoint" but forecast: "2018 should see an uptick from net new business wins."