Lance at Large was screened on the BBC in the 1960sA film reel found in the basement of Coventry Cathedral has been confirmed as an episode from a BBC TV comedy series.
Lance at Large was screened in the 1960s, featuring popular entertainer from the time, Lance Percival.
Staff at the cathedral said they were mystified as to how it got there and were trying to find out more.
Dick Fiddy, archivist with the British Film Institute, said: "It never ceases to amaze me where things do turn up."
The reel was found in a cupboard in a room where religious programmes were once made.
On the film's tin, there is an Australian broadcasting label on it, and one theory from staff is that it was a syndicated copy brought back from the country.
Dianne Morris, archivist at the cathedral, said: "I'm not aware of any people visiting to bring the film back with them, so unless it was someone from the BBC themselves who brought it and maybe put it down and forgot to take it away with them, we'll never know."
Percival, who died nine years ago, was involved in many TV programmes and films including Carry On Cruising, and That Was the Week That Was.
His son Jamie has travelled from London to visit the cathedral to see where the missing episode was found.
"It brings back memories of him as a dad and as a performer," he said.
"When I was actually watching it, I personally could see what I would say is my dad's personality, what he was like at home, and also the personality he would have when he was actually doing a performance, when he was being an entertainer.
"Dad had me when he was over 50, so he was an older dad but nonetheless gave it as much energy as any dad of any age would try to, so I had a very, very good time growing up with him."
The British Film Institute (BFI) has ratified the copy and featured it in a recent Missing Believed Wiped festival.
"It's extraordinary," Mr Fiddy said.
"We have no idea what it was doing in Coventry Cathedral and things like this, when it's the only episode of a completely missing series, I think that has even more significance."
The episode has now been handed back to the BBC and will sit alongside the many other programmes the actor was involved in.