The amount spent on online advertising has slowed to under 20 percent in the most mature markets, as search remains the most popular online advertising format, according to new research. The findings, from the Interactive Advertising Bureau Europe (IAB Europe), form part of its annual advertising expenditure survey for the year ending December 2008. The 2008 report contains online expenditure data for 19 markets. In 2008 the European online advertising market was worth EUR 12.9bn with a like-for-like growth rate compared to 2007 of 20%. In the US, online advertising grew 10.6 percent in 2008 and was worth EUR 16.6bn. However whilst the pan-European figure is one of growth, it is no secret that 2008 was one of the worst years for advertising in any medium. The online sector was not immune and experienced a challenging year, particularly in the 10 most mature markets. The key difference is it did still manage growth - albeit at a lower rate. The top 10 markets in Europe account for 93 percent of the total value of the market. Many of these countries saw year-on-year growth rates under 20 percent. The Netherlands, Europe's early adopter of online advertising grew only 9 percent in 2008. Other mature online markets saw similar trends, with growth slowing in France at 18.5 percent; the UK, Germany and Sweden at 19 percent and Italy at 20 percent. Just beating the 20% barrier, Belgium grew 21 percent and Denmark and Norway hit a 22 percent growth rate. Spain by comparison saw relatively strong growth at 26 percent. Search remains the leading format in Europe with the strongest year-on-year growth rate of 26 percent, accounting for 43 percent of online ad expenditure in the 19 countries measured and a value of EUR 5.6bn.