However, it still boosted profits and sales in the first six months of the year even though it said it had been "ravaged" by a strong pound.
The world's largest ad group said headline pre-tax profit was up 1.5 per cent at £532 million, while sales were three per cent higher at £5.5 billion.
WPP, which employs over 179,000 staff, expects 2014 to be "another demanding year" as a strong pound impacts currencies in faster growing markets in Asia and South America.
The company, led by founder and chief executive Sir Martin Sorrell, said a fragile eurozone, an unstable Middle East, the US budget deficit, crisis in the Ukraine and political decisions in the UK over its membership of the EU and Scottish devolution all created uncertainty.
WPP said this led its client firms to reach "profitability objectives by cutting costs, rather than by growing the top line."
The marketing services group added: "Advertising as a proportion of gross domestic production should at least remain constant overall, although it is still at relatively depressed historical levels, particularly in mature markets."
The group's UK agencies include Burson-Marsteller, Ogilvy and RLM Finsbury.
WPP reported that billings - the amount an agency charges its clients - were down three per cent at £22.1 billion but once sterling translations are stripped out the figure rose 5.7 per cent. The group forecast like-for-like sales would grow at over three per cent in 2014.
In the UK, the firm posted sales up 17.2 per cent to £784m as it grew in public relations and media investment management, which offset weaker sales among its data management and healthcare communications arms. In the US, sales lifted 2.1 per cent to £1.9bn but in continental Europe half-year sales slipped 1.1 per cent to £1.2bn as difficult markets offset strong growth in Spain, Portugal and the Netherlands.