British pay-TV rivals Sky and Liberty Global's Virgin Media will team up to offer advertisers access to more than 30 million TV viewers in Britain and Ireland.

The partnership, which covers both broadcast and video on demand (VOD) advertising, will make use of Sky's AdSmart targeted advertising platform as well as Liberty Global's technology, Sky said.

AdSmart enables Sky to provide different ads for different households watching the same programme, allowing advertisers to target viewers according to factors such as age, lifestyle and location using Sky's customer data.

TV broadcasters are fighting to preserve their share of the ad market as companies spend more on social networks such as Facebook and online platforms including Google's YouTube.

European broadcasters TF1, ProSiebenSat.1 and Mediaset said last week they would set up a joint trading platform for digital video advertising to appeal to media agencies planning continent-wide campaigns.

Sky said the partnership with its rival would give companies access to more than 30 million "targetable" TV viewers in Britain and Ireland, an audience on par with social networks such as Facebook.

Sky's Chief Operating Officer Andrew Griffith said thousands of leading brands were tailoring their campaigns by using AdSmart.

"Today's partnership takes that to the next level with the extension of AdSmart to millions more homes meaning more relevant ads for Virgin customers and a larger platform for advertisers," he said.

Spot advertising on TV is forecast to grow 0.7 percent this year, according to eMarketer, while digital advertising is forecast to grow 9.5 percent.

Video-on-demand advertising, however, is growing rapidly, with eMarketer predicting a 12.4 percent rise in ad spend.

Television retains two advantages over some online platforms, namely the transparency of viewing numbers and content that has to meet strict broadcasting standards.

Some large brands have pulled ads from YouTube over concerns that their ads may have appeared on channels that broadcast offensive videos.

Facebook admitted last year it overstated some key ad metrics on its site, and it is taking steps to provide more clarity to advertisers.

Sky said the tie-up with Virgin gave brands the ability to target audiences whilst utilizing the "attractive, quality, trusted, and brand safe environment of TV".