The Superman memorabilia came into the charity’s possession as a thank you gift from Cannon Films, which produced the film.
The key was signed by the late Gene Hackman who played the character Lex Luthor and the late Margot Kidder who played Lois Lane.
It was valued at £2,500, although the auction house, Goldin Auctions in the United States, believed it could fetch a higher price because of its “unusual provenance and connection to such an iconic Hollywood film and its stars”, according to Mark McCarthy, a spokesperson for the charity.
“Our ultimate dream would be for a local Milton Keynes business or individual to purchase the key at auction and bring it back home,” McCarthy said.