Image caption Spamalot took more than $1m in its opening week on Broadway

The surviving members of Monty Python are being sued by one of the producers of cult film Monty Python and the Holy Grail over royalty rights to Spamalot.

Hit musical Spamalot is described on posters as being "lovingly ripped off" from the 1975 film Holy Grail.

But Holy Grail producer Mark Forstater claims he has been underpaid royalties since the show's launch in 2005.

Eric Idle, Michael Palin and Terry Jones are due to give evidence at London's High Court in the coming days.

John Cleese and Terry Gilliam, who are abroad, are not expected to attend the five-day legal battle. The sixth member of the comedy troupe, Graham Chapman, died in 1989.

The claim is being brought by Mr Forstater and his company Mark Forstater Productions Ltd against Python (Monty) Pictures Ltd (PMP), which represents the film interests of the Python team, and Freeway Cam (UK) Ltd, which holds the copyright in the Holy Grail as trustee for those entitled to profit from it.

Mr Forstater's lawyer, Tom Weisselberg, told the judge on Friday that "for financial purposes", Mr Forstater should be treated as "the seventh Python".

He accused the Pythons of "failing to pay Mr Forstater monies he says are owed to him under an agreement reached with PMP back in 1974".

Both sides agreed that Mr Forstater was entitled to a share in merchandising and spin-off income, but dispute the extent of his entitlement.

Image caption Mr Forstater was declared bankrupt earlier this year.

Under the terms of the 1974 agreement, investors in the film, such as Mr Forstater, were entitled to a share in 50% of all merchandise revenues.

Mr Forstater claimed he was entitled to one-seventh of this figure, the same share enjoyed by each of the other Pythons - but was told he was only entitled to one-fourteenth, and has been paid accordingly since 2005.

"Mr Forstater is in difficult financial circumstances and has been forced to bring these proceedings," Mr Weisselberg told the court.

The film producer was made bankrupt in June, but last month, the bankruptcy was annulled and he entered an independent voluntary arrangement (IVA) to deal with his debts.

Written by Eric Idle, Spamalot premiered on Broadway in 2005, winning three Tony Awards. A critical and commercial success, it took $1m in its opening week on Broadway.

Tim Curry won acclaim for his turn as King Arthur and went on to debut the show in London in October 2006.

Like the film upon which it is based, the stage comedy is essentially about a group of medieval knights searching for the mythical Holy Grail but the plot broadens out to spoof Broadway, and various musicals, including those of Andrew Lloyd Webber.

"There has never been a sillier musical than this, or one more calculated to appeal to the British sense of humour," wrote Telegraph critic Charles Spencer, at the UK premiere.