M&M'S chocolates has opened a four-storey megastore in London dedicated to the guilty treats alone.

London M&M'S World store, in Leicester Square, on 3,250sqm of floor space in the old Swiss Centre Building, is the stuff of dreams – and perhaps nightmare weight gain.

Launched on Wednesday by London Mayor Boris Johnson, and featuring performances from JLS and Alexandra Burke, it is expected to become a must-see attraction in the popular tourist area, selling clothes, kitchenware and bedding – as well as a lot of chocolate.

Visitors enter the £10m store through colourful window displays and will immediately be greeted with something of a feast for the senses.

Chocolate-lovers will encounter seven-foot statues of the various M&M'S characters and one of London’s iconic 1963 double-decker buses.

In the middle of the store, a sweeping red staircase is illuminated by a vast multi-coloured M&M's chandelier and an M&M's union jack flag.

However, chocolate fiends are bound to get into a fizz over the store’s biggest attraction – the Wall of Chocolate, where visitors can create their very own custom-blended M&M'S selection from a range of 22 colours of M&M'S which are housed in more than 100, gigantic clear tubes.

M&M'S fans can pose for photos with the characters throughout the store or while walking on an in-store 'zebra crossing', they can even be visited in the M&M'S Character Worlds, as Red, Yellow, Green, Blue, and Orange are brought to life in themed, personalised areas.

JLS Singer Marvin Humes said: “It’s madness down here. We are literally kids in a candy shop.

“It’s just amazing. Kids will love it and adults as well.”

The store also houses a walk-through time line of M&M's history, with characters dating back to 1954 and a room full of M&M's mixologists hard at work.

Mayor of London, Boris Johnson commented saying: "Europe's first M&M World is sure to become one of the capital's tastiest attractions."

The nutty shop is the first of its kind in Europe and is similar to stores in New York, Orlando and Las Vegas, which pull in more than 10million people a year.

Susan Saideman of Mars, which makes the sweets, said the store was a ‘totally unique shopping experience’.

The M&M'S World store will be open daily from 10am – midnight, and from 12 to 6pm on Sundays.