Alec Guinness
Professor Marcus

Cecil Parker
Claude, alias 'Major Courtney'

Herbert Lom
Louis, alias 'Mr. Harvey'

Peter Sellers
Harry, alias 'Mr. Robinson'

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

Katie Johnson
Mrs. Wilberforce

Jack Warner
The Superintendent

Philip Stainton
The Sergeant

Frankie Howerd
The Barrow Boy

Kenneth Connor
Cab Driver (uncredited)

Harold Goodwin
Parcels Clerk (uncredited)

Lucy Griffiths
Miss Pringle (uncredited)

Stratford Johns
Security Guard (uncredited)

Sam Kydd
Second Cab Driver (uncredited)

Edie Martin
Lettice (uncredited)

Jack Melford
Detective (uncredited)

Arthur Mullard
(uncredited)

Ewan Roberts
Constable (uncredited)

Peter Williams
Detective at Parcels Office (uncredited)

Neil Wilson
Policeman (uncredited)