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The high street supermarket is known for its friendly cake character, Colin. Now, it's claiming Aldi's own version is too similar, and "rides on the coat-tails" of its reputation.
M&S lodged an intellectual property claim with the High Court this week, and is asking Aldi to take the cakes off shelves, and agree not to sell anything similar.
The Colin the Caterpillar cake is protected by three trademarks.
The iconic product was invented 30 years ago, and has hardly changed since 2004 - apart from seasonal adaptations, around Christmas, Valentine's Day and Halloween, for example.
The cake was also used as a basis for the store's second Caterpillar character, Connie - dubbed Colin's life-partner.
The cake is essentially chocolate sponge with milk chocolate and buttercream. It's decorated with a hard chocolate shell, sweets, cute little feet and a recognisable white chocolate face, too.
A spokesman for the supermarket said: "Because we know the M&S brand is special to our customers and they expect only the very best from us, love and care goes into every M&S product on our shelves.
"So we want to protect Colin, Connie and our reputation for freshness, quality, innovation and value."
Since M&S launched the Colin cake, other supermarkets have followed suit.
ASDA launched a caterpillar called Clyde, Waitrose invented Cecil, Tesco's got Curly and Sainsbury's now sells Wiggles.
The Colin cake jar is described as being filled with "layers of all the Colin goodness", which is basically a rich chocolate sponge, Belgian chocolate sauce, chocolate ganache, milk and white chocolate chips and and a Colin the Caterpillar white chocolate face.
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