WPP, the world's biggest advertising group, has raised fresh fears of a global economic slowdown as it warned of a marked change in behaviour by clients since September.
Sir Martin Sorrell, chief executive, said it was clear that sentiment had worsened, pointing to big cuts by the car-maker Ford, which axed 1,300 UK jobs yesterday, and weak results by the Swiss banking giant Credit Suisse.
Sir Martin explained he has spoken to the bosses of more than two dozen major multinationals in recent weeks and "people have just become very, very cautious".
He said: "We don't know why. Most clients saw a significant change in September. It could be people came back from their holidays and looked at the rest of the year and were cautious."
The WPP chief added that it was "correct" to describe this as a global slowdown.
He said four macroeconomic factors, which he called "four grey swans", were affecting client behaviour: the eurozone crisis, volatility in the Middle East, a soft landing for the Chinese economy and the US fiscal deficit.
The swan reference comes from the academic Nassim Nicholas Taleb's book The Black Swan, in which he talked about known factors called "grey swans" and rare, unpredictable factors such as a "black swan".
WPP's own sales growth ground to a halt last month, after increasing by 3 per cent in July and August and 3.6 per cent during the first half of the year.
"September was flat," said Sir Martin, describing it as "significantly different from July and August".
Overall, group sales rose 1.9 per cent in the quarter to £2.5bn. The UK performed well, jumping 4.7 per cent. Asia and Latin America were also strong. But the US fell 0.4 per cent and Europe 2.1 per cent.
WPP shares were the third worst fallers on the FTSE, dropping 2.3 per cent or 18.5p to 789.5p. The group cut revenue and profits forecast for 2012.