Research firm eMarketer has forecast that growth in US online ad spending will slow slightly this year as due to weaker consumer spending and as the market matures.

A report by the firm predicted internet ad revenues would rise 26.8% to $15.9bn this year – a move that may calm some fears in the industry after Yahoo!’s warning sent shares tumbling last week.

But that represents a slight cooling off from growth rates of 30% and above in the past two years, and is lower than eMarketer’s previous estimate of $16.7bn, the company said.

eMarketer also lowered its 2006 US ad sales growth estimates for Google from 80% to 65% and for Yahoo! to 17%.

The internet will still be growing at a faster rate than television and most other media, but it’s going to be coming back down to sustainable level,” said eMarketer chief executive Geoff Ramsey.

Overall, the study said US online ad revenues would hit $25.2bn by 2010, by which time growth rates would have slowed to 6.8%.

Meanwhile, the Interactive Advertising Bureau and PriceWaterhouseCoopers have released a more positive report which estimates online ad spending grew by 37% in the first half of the year, reaching a record of $7.9bn.

Search advertising revenues increased 40% in the six months, while classified rose 20%, the study said.

However, one area where that appears to be less the case is Hollywood, with ClickZ reporting on a Google-funded study which indicates that only 2.6% of movie studios’ marketing budgets are spent online.