Puckoon, a black comedy boasting a cast led by Richard Attenborough, faces calls from UK actors union Equity for its public sector backers to drop out because of a dispute over cast contracts.

In a statement, Equity said it will demand that public film bodies, the Northern Ireland Film Commission and the Northern Ireland Arts Council, divorce themselves from the production because actors are not on formal equity contracts. Although not contractual, the commission's code of practice states that agreements between Equity and producers body PACT "should form the basis of all cast employment."

Equity is to call for the UK minister of culture to step in because of clauses in the contracts relating to several areas including remuneration and insurance.

But the film-makers pointed out that they are not obliged to use official Equity contracts. "In this day and age you don't have to be in any union," said producer Ken Tuohy. "That doesn't mean what we have agreed to doesn't abide by common practice."

The adaptation of Spike Milligan's novel about an Irish village cut in two by the north-south border also includes Elliott Gould and Sean Hughes. "Nobody has been abused in any shape, way or form," said Tuohy.

The dispute is embarrassing for the film commission, which heralded the production as the first feature to use Belfast's Paint Hall Studios. Richard Taylor, the commission's chief executive, said no one had complained to the commission.

He expects Puckoon, now in the final stages of a six-week shoot in Northern Ireland, to spend at least £1m in the country. "Puckoon is very good news for Northern Ireland," he said.