WPP has reported strong results for the first half of 2008 with like-for-like revenue up 4% to £3.3bn and pre-tax profits up 15% to £339m - but the world's second largest advertising group has warned that 2009 looks "uncertain".

WPP said that its performance in 2008 should match that of last year, despite economic uncertainty, due to major advertising drivers such as the US presidential election and the Beijing Olympic games.

WPP's performance in the first half of the year, a marked improvement after Sir Martin Sorrell's company saw a deterioration in growth at the end of the first quarter, was fuelled by a strong performance in emerging markets such as Asia.

"The group's profit performance in the first half of the year was strong and ahead of most estimates, despite the general economic tightening in the United States and western Europe in the second quarter," the company said.

Revenues in the six months to the end of June were up 2.1% in North America, 2.5% in the UK and 3.2% in WPP's continental Europe operation.

In emerging markets such as Latin America and Asia, the company's revenues were up 10.7% in the first six months.

Overall, WPP managed to increase its operating margin by 0.5% to 13.6%.

The stronger-than-expected results led WPP to increase its dividend by
20% to 5.19p per share.

However, WPP posted a note of caution about performance in the medium term.

"The prospects for 2009 remain less certain, particularly if the United States and western European economies continue to be impacted by the financial crisis and commodity price increases," the company said.

WPP said the Chinese economy, a massive engine of growth for advertising, may slow due to inflationary pressures and the impact of the weakening US economy on the rest of the world.

The company said it expected the economic difficulties to remain until the middle of next year, with prospects for 2010 looking good.

"In mid-2009 the financial markets may start to rebound and 2010, in the real world, continues to look better," said WPP.

"[2010 has] the prospect of the impact of 'mini-quadrennial' events such as the Fifa World Cup in South Africa, the Winter Olympics in Vancouver, the mid-term congressional elections in the United States, the World Expo in Shanghai and the Asian Games in Guangzhou."

· To contact the MediaGuardian news desk email editor@mediatheguardian.com or phone 020 7239 9857. For all other inquiries please call the main Guardian switchboard on 020 7278 2332.

· If you are writing a comment for publication, please mark clearly "for publication".