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Viewers in the US will now start to see a transparent overlay at the bottom of video clips. If users decide to click on the overlay, the full advert will be played, otherwise it will disappear after 15 seconds.

Google, which has tested several methods of video advertising, claims this method is less intrusive than the pre-rolls which force users to watch an advert before the clip begins.

"The philosophy at YouTube is pretty much core to what we at Google do generally, which is that all the ads we serve need to provide value to the end user," says Eileen Naughton, Google's director of media platforms. "Our advertising partners are eager to use YouTube as a branding platform."

The scheme is expected to be rolled out to British viewers within months. The adverts will only be shown on videos from YouTube's commercial partners - such as the BBC and Chelsea FC - in the initial phase.