The 'Union Jack' TV ad, created by Abbott Mead Vickers BBDO first aired yesterday (27 May), and will run for four weeks in a media plan handled by MediaCom.
It shows the M&M yellow and red brand characters in a lounge, with a street party scene behind them.
Yellow M&M is trying on a range of outfits, including those of a punk and morris dancer, in a bid to make him look "British enough" to make it into the special edition packet.
The ad closes with the strapline, "Red, white and blues only, sorry yellow".
Mars, which owns brands including M&Ms, Skittles, Pedigree, Whiskas, Uncle Ben's and Wrigley, was recently named Cannes Lions Advertiser of the Year.
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