The format enables a semitransparent ad to appear on a strip at the bottom of the video and it appears after a video plays for 15 seconds.

If the user who watches the video doesn’t click the ad it disappears up to 10 seconds later. The ad format is non-intrusive because the viewer can click to close the ad right away. In case he chooses to watch the ad, the main video pauses until the commercial stops.

"We tested and trialed a number of advertising formats," said YouTube director of media and platforms Eileen Naughton. "We needed to be respectful of the community while offering something rich and engaging as a marketing platform."

"Ads need to provide value to the user community," said Eileen Naughton. "We've proved over and over again on Google that ads are really useful information when users raise their hands and engage with them."

According to the company, for the moment the ad format will only run on video content provided by select partners and not on videos uploaded by individuals.

YouTube has already established revenue-sharing deals with more than 50 partners, including Ford Models, Warner Music Group Corp, News Corp.'s 20th Century Fox and Time Warner Inc.'s New Line Cinema.

Marketers can target their ads by user demographics, location, time of day or genre, such as music videos or sports

YouTube started testing its in-video ad format in June and July on more than 200 videos from 20 content providers, and found that 75 per cent of viewers watched the entire ad. The ads had five to 10 times greater click-through rates than standard display ads that appear on Web sites, YouTube said.

They were also much more successful than so- called pre-roll ads, which appear prior to the screening of the video and caused some 70 per cent of viewers to abandon viewing the clip.