Robbie's Pepsi challenge

Williams is donating money from the ads to charity

Pop star Robbie Williams has taken a tongue-in-cheek swipe at lip-synching singers in his first TV ad for Pepsi.

The commercial goes out for the first time on Friday on Channel 4 and sees Williams chatting to a backstage reporter at a concert.

Asked by the journalist what he thinks of singers who mime to their songs, Williams replies:

"Miming? It's disgusting, You'd never see me doing that."

Then, as he turns away from the camera to take a swig of Pepsi, Williams can still be heard speaking, revealing that his response has itself been dubbed.

Charity

Williams- whose latest single Rock DJ is out on 31 July - has written a new song for the soft drink campaign called United.

It will be made available on CD-Rom in exchange for drink can ring-pulls, following a deal between Pepsi and the home entertainment retailer HMV.

Williams joins a long line of stars who have promoted Pepsi

Williams has had full creative control over the ad - part of a worldwide campaign for which Williams acts as frontman.

He has signed a seven-figure sum with Pepsi believed to be in the region of £2m.

But Williams - who has acted as a goodwill ambassador for the United Nations' Children's Fund, Unicef - is to donate money from the campaign to his own charity Give It Sum.

Williams is the latest in a string of pop stars to feature in Pepsi ads, including Ricky Martin, the Spice Girls, Michael Jackson, David Bowie and Boyzone.