The robe, which was missing for 28 years, was found during a clear-out at Angels Fancy Dress shop and is estimated to be worth tens of thousands of pounds.
Guinness wore the iconic cloak for the first of George Lucas' blockbuster Star Wars series in 1977. Since then it has been worn by an extra on The Mummy and probably used in The Name of the Rose, starring SIR Sean Connery.
TIM ANGEL, who runs the costume company with his daughter, was astonished to find the film relic amongst six miles of clothes.
He said, "One of the guys found a brown robe. One of my managers said: 'Oh my God, that is Alec Guinness's cloak.'
"It was a fluke that we found it. When you look at this very old brown fabric you wouldn't think much of it."
The robe is to be displayed in an exhibition of film costumes at Harrods.