YouTube has written to certain content producers to warn them against the practice of putting their own ads in their videos, according to Media Week.

The publication reports that YouTube has identified a number of content providers that have formed deals with advertisers to place their ads within videos, rather than these advertisers using YouTube's own ad network to promote their wares.

In its letter, YouTube states that including unauthorised ads in videos goes against its terms of service.

Officials at YouTube revealed to Media Week that a specific policy regarding brand integration videos is to be launched in due course.

Commenting on speculation that YouTube may take down offending videos if producers fail to cooperate with it, Tom Pickett of YouTube said: "We have tried to take a more hands-on approach to see if we can facilitate deals. But they shouldn't be fearful of getting booted off the site."

Media Week said that YouTube's letter-writing strategy may be part of efforts to generate more revenues from the site.

Spencer Wang, analyst at Credit Suisse, recently suggested that YouTube is likely to incur $470.6 million (£312 million) in losses in 2009 due to rising costs.