The producer of the film Monty Python and the Holy Grail has sued the Pythons for what he believes is his rightful share of the proceeds from the spin-off musical Spamalot.

American-born Mark Forstater, who has lived in Britain for more than 40 years, was instrumental in bringing the Pythons’ Arthurian spoof to the screen in 1975.

But he believes he could be owed at least £250,000 from the merchandising proceeds from the film, which include the Spamalot stage show.

According to Mr Forstater, as well as a share of the profits from the actual film, he was entitled to a cut from any merchandising.

He says that half of the merchandising profits were to go to investors in the Holy Grail, among them rock bands Pink Floyd and Led Zeppelin.

Eric Idle, Terry Jones and Michael Palin gave evidence

The other half was to be split seven ways among the six Pythons and himself, Mr Forstater claims.

But the Pythons argue that Mr Forstater was entitled only to one 14th share of their merchandising profits, not a seventh.

At first, Mr Forstater benefited from ventures such as a book and a record based on the Holy Grail but it was the musical Spamalot that has proved the most lucrative spin-off from the film, grossing more than £100 million on Broadway, the West End and elsewhere since it opened in 2005.

From 1976 to 2005, Mr Forstater received his one-seventh cut of the Pythons’ merchandising share. Lawyers for the Pythons, however, say that this was a mistake and the correct amount should have been one 14th.

But lawyer Simon Olswang, who was acting for the Pythons at the time of the Holy Grail, has backed the producer’s version of events, saying that he was sometimes called the “Seventh Python” .

Three of the five surviving Pythons — Eric Idle, Terry Jones and Michael Palin — gave evidence in person at a five-day hearing which concluded last week, before Mr Justice Norris.

Lawyer Laurence Abrahamson, representing Mr Forstater, said: “The case turns on the wording of the contract and what the parties had in mind at the time. We expect to hear the judgment by the end of March.”

Mr Forstater first met the Python’s animator Terry Gilliam in New York in the 1960s and later renewed contact with him in London after noticing his name among the credits for a television show Do Not Adjust Your Set which pre-dated Monty Python.

While he awaits the outcome of the court case, Mr Forstater is busy with the forthcoming publication of a book on the Shoah, How I Survived A Secret Nazi Extermination Camp. It was inspired by the testimony of Rudolf Reder, one of the few survivors of Belzec, where several of Mr Forstater’s relatives perished.