Burgeoning interest in the treat, which is made from two discs of sponge filled with buttercream, prompted the retailer to bring forward the launch date of the range to coincide with the first May Bank Holiday.
Over 50 M&S stores sold out of whoopie pies in the first two days, with emergency deliveries needed to meet demand on the Bank Holiday Monday.
"Whoopie pies are to 2010 what the cupcake was to 2009," said M&S product developer Chris Seaby. "We brought the launch forward two months, because we knew they would be popular, but the response has been phenomenal."
Four-packs of the pies retail for £1.99 and are available in three flavours: chocolate with vanilla; vanilla with chocolate; and toffee.