Sainsbury's is inviting fans of its Christmas ad campaign, The Greatest Gift, to sing along with the vocals today through the U.K.'s first branded "karaoke" lens on Snapchat -- and they can also win a chance to have their singing featured in a final version.

The Snapchat lens, live for 24 hours today (Dec. 2) turns users into a lookalike of the "Dave" animated character from the ad, and and allows them to share their version of the song, sung by James Corden in the TV version.

To wrap up the campaign, the best karaoke vocals from across the festive period will replace James Corden's voice in a special version of the TV ad released online in the week before Christmas. People can also enter the competition via social channels.

Analog Folk created the interactive element of the Sainsbury's campaign; the TV ad is by AMV.BBDO.