Before “Monty Python’s Flying Circus” made John Cleese and Graham Chapman international television comedy stars, they – along with Marty Feldman, Tim Brooke-Taylor and Aimi Macdonald – wrote and starred in “At Last the 1948 Show,” a less widely traveled British satirical program.

The first and last episodes of the show, long thought to have been lost, were recently found in the private collection of David Frost, the television host and impresario, and the show’s executive producer, who died in 2013, the British Film Institute announced on Thursday.

(A complete show, labeled Episode 1, has been on YouTube since 2012. A call to the British Film Institute for clarification was not immediately returned. However, excerpts from the newly found shows provided to the BBC were not included in the YouTube episode, and are significantly better quality.)

The institute said that the discovery was made by Dick Fiddy, its television consultant, who had been invited by the Frost family to explore Frost’s collection. The show had its roots in Frost’s admiration for the humorous sketches Mr. Cleese had written as a member of Cambridge Footlights, at Cambridge University, in the early 1960s. Mr. Frost asked Mr. Cleese and company to build on that approach, and they did, for two seasons – 13 episodes in all – in 1967 and 1968.

Mr. Cleese and Mr. Chapman expanded the style further in Monty Python, which began in 1969, and even used reworked versions of some “1948 Show” material, including the “Four Yorkshiremen” sketch, in which a group of men outdo each other in describing the deprivations of their childhoods.

“This latest recovery is a crucial find,” Mr. Fiddy said in a statement. “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.”

The master tapes of the show were long believed to have been erased in the late 1960s, when the archives of Rediffusion London, Frost’s partner in the production of the shows, was acquired by Thames Television. But two episodes are said to have been saved by Mr. Cleese, and five others were found in the archives of Swedish television. Mr. Frost’s episodes bring the number of complete shows to nine, and excerpts from the remaining four have survived as well. The material is now part of the institute’s collection.

The two newly discovered episodes will be screened at the Southbank Center in London on Dec. 7, as part of the institute’s annual “Missing Believed Wiped” series devoted to recently found television shows.