Two long-lost episodes of a TV series regarded as a forerunner of Monty Python will be shown again after they turned up in the collection of the late Sir David Frost.

The first and last episodes of At Last The 1948 Show, which starred future Pythons John Cleese and Graham Chapman, have not been seen since they were broadcast in 1967.

The show, which also starred Tim Brooke-Taylor and Marty Feldman, were made by Frost's production company and discovered after his family invited archivists to examine his private collection.

They were on reels of 16mm film and had been filmed directly from a television screen.

British Film Institute television consultant Dick Fiddy, who discovered the tapes, said they were "a crucial find".

He said: "It represents a key moment in the history of British television comedy featuring the combined talents of some of its greatest exponents.

"These gifted comedians, all in their 20s and 30s, were let off the leash and allowed to experiment with style and content, resulting in shows which have had an enduring influence on comedy worldwide.

"Even very recently, the famous Four Yorkshiremen sketch - which originated on At Last The 1948 Show - was used as the opener for the Python's stage shows at the 02 and had been performed previously by the team in galas such as the Secret Policeman's Ball.

"Cleese, Feldman, Brooke-Taylor and Chapman, created, scripted and starred in the 1948 Show and the fact that the show remains very, very funny 47 years later is a tribute to their extraordinary abilities."

Frost, who died last year, got his big break when he co-created and hosted satirical show That Was The Week That Was in the early 1960s.

Another of his early programmes, The Frost Report, effectively launched John Cleese, Ronnie Barker and Ronnie Corbett on their glittering careers.

John will present a screening of the episodes of the sketch show at BFI Southbank on Sunday December 7.