The secret of Marilyn Monroe’s most outstanding asset can finally be revealed nearly 50 years after her death.

The iconic actress, famed for her curvaceous figure, wore the 1950s version of a Wonderbra, which enhanced her cleavage, boosted her cup size and added more than an inch to her bust measurement.

The cunningly-designed bra had under-wiring and four cups, an inner pair to hold her breasts and an outer pair an inch in front of the real ones to enhance her bust. In the days before surgical implants, it was one way to get herself noticed.

The bra, or one very similar, is believed to have been worn in The Seven Year Itch, when she appeared in a white halterneck dress. The straps would have been concealed beneath the halter, allowing her audience to believe her shape was entirely natural.

The cream and ivory-coloured bra was part of the wardrobe of Monroe’s clothes sold in Hollywood in 2005. It is being auctioned in London on Saturday, when it is expected to fetch around £2,000.

Katie Halford, a designer for the vintage lingerie firm What Katie Did, said the style was known as a “fling” bra and was a favourite with burlesque artistes.

She said: “They were a bit naughty. They were worn by strippers and burlesque artists in the 1940s and 1950s. Marilyn Monroe didn’t like wearing underwear and, if she could get away with it, she wouldn’t wear any.

“It would have given her breasts extra support and uplift and left her nipples exposed.

“It would have given the impression that she had very good and perfectly positioned breasts, while making it appear she wasn’t wearing a bra at all, and because it is a bit padded, it would have made her breasts look a bit bigger.”

Richard Davie, from International Autograph Auctions, which is selling the bra, also expects a pair of Monroe’s unworn Dior silk stockings to fetch up to £1,200 in the sale at Radisson Edwardian Hotel, Heathrow.

A signed black and white photograph of the star holding a bunch of roses is also expected to attract bids of £3,000.

Monroe died in 1962 from a drug overdose.