Alec Guinness as Professor Marcus

Cecil Parker as Claude, alias 'Major Courtney'

Herbert Lom as Louis, alias 'Mr. Harvey'

Peter Sellers as Harry, alias 'Mr. Robinson'

Danny Green as 'One-Round', alias 'Mr. Lawson'

Katie Johnson as Mrs. Wilberforce

Jack Warner as The Superintendent

Philip Stainton as The Sergeant

Frankie Howerd as The Barrow Boy

Madge Brindley as Large Lady (uncredited)

Hélène Burls as Hypatia (uncredited)

Kenneth Connor as Cab Driver (uncredited)

Michael Corcoran as Burglar (uncredited)

Harold Goodwin as Parcels Clerk (uncredited)

Fred Griffiths as Junk Man (uncredited)

Lucy Griffiths as Miss Pringle (uncredited)

George Hilsdon as Security Van Driver (uncredited)

Vincent Holman as Station Master (uncredited)

Stratford Johns as Security Guard (uncredited)

Sam Kydd as Second Cab Driver (uncredited)

Edie Martin as Lettice (uncredited)

Jack Melford as Detective (uncredited)

Arthur Mullard as (uncredited)

Ewan Roberts as Constable (uncredited)

George Roderick as Radio Car Policeman (uncredited)

John Rudling as Nervous Man (uncredited)

Leonard Sharp as Pavement Artist (uncredited)

Peter Williams as Detective at Parcels Office (uncredited)

Neil Wilson as Policeman (uncredited)

Phoebe Hodgson as Constance (uncredited)