Morace Park was footling around on eBay looking for antiques when he stumbled on an item that was listed casually as an "old film" - and even then he was really more interested in the tin it was in.

"It had a lovely look to it," said Park. But the contents of the battered container, which he bought for the princely sum of ?3.20, have turned out to be a previously unknown film by Charlie Chaplin.

Park - who, when he is not buying and selling antiques as a hobby, runs a company that develops products with inventors - bought the film "from someone else who deals in bits and bobs". When his parcel arrived, he didn't even bother to open it for a while. But when he did, he unfurled a little of the film and saw the title: Charlie Chaplin in Zepped. "I Googled it," he said, "and then my interest was pricked. I couldn't find any sign of it on the internet."

Park, from Essex, enlisted the help of a neighbour, John Dyer, the former head of education for the British Board of Film Classification. Dyer's excitement on seeing the film was "the catalyst for a wild journey" as the pair turned detective to try to unravel the mystery of what Zepped was - and why it was completely unknown to film historians and Chaplin experts.

The film, just under seven minutes long, is a mixture of footage of Chaplin and exuberant animation that reminded Park of Monty Python sequences. "It starts with live shots of Chaplin. It then turns into a dreamscape. We see a Zeppelin bombing attack. And then we see Chaplin taking the mickey out of the Zeppelin, at the time a powerful instrument of terror," he said.

They concluded that the movie, shot on 35mm nitrate film, had been put together as a first world war propaganda piece aimed at defusing fear of airship bombing raids, which had been launched on Britain by Germany from the beginning of 1915.

Another clue to the film's date and origins was provided by a reference in early frames to the Essanay film company. The 25-year-old Chaplin was contracted to the California-based company in December 1914, making such early masterpieces as The Bank, Work, and The Tramp, which established his "little tramp" character. But a year later, disputes over his contracts and salary led to a severance of relations between the star and his employers.

Park and Dyer are currently in Los Angeles to find out more, accompanied by the film-maker Hammad Khan, who is making a documentary about their quest.

On Monday, they showed the film to Michael Pogorzelski, a film-history expert and director of the archive of the Academy of Motion Picture Arts and Sciences, the body responsible for the Oscars. "It is an extremely interesting find," he said. "An unknown and uncatalogued Charlie Chaplin film."

Pogorzelski believes the film consists of outtakes and footage from previous films re-edited by Essanay, and spliced together with fresh shots of Zeppelins and animated material, to create a "new" film. It was, he said, "definitely important and definitely interesting". It was an example of what he called "either piracy or entrepreneurship - depending on which side of the fence you're on."

According to David Robinson, the author of Chaplin: His Life and Art, when Chaplin left Essanay, the company tried to exploit the footage it had, adding two further reels to Burlesque on Carmen, a film Chaplin had completed as two reels, to create a film of feature length. This resulted in a volley of litigation and counter-litigation.

Robinson - who has not yet seen Zepped - believes the film may fall into this context, with Essanay attempting to get maximum mileage out of its lost star. The legal controversy may account for the fact that Zepped never saw wide circulation.