SEOUL (AFP) - A South Korean cinema chain under pressure from conservatives on Saturday withdrew a controversial documentary film challenging government findings that a warship sunk in 2010 was torpedoed by the North.

It was the first time in South Korea that the screening of a film has been cancelled under political pressure, Yonhap news agency said.

Megabox, a leading chain of multiplex cinemas, said it had decided to stop screening Project Cheonan, a 75-minute documentary exploring possible causes of the sinking that run counter to the government conclusion that the corvette was torpedoed by North Korea.

Pyongyang flatly denied responsibility for the sinking which claimed the lives of 46 sailors. But South Korea responded by cutting off almost all ties with Pyongyang.

"We've decided to withdraw the film out of concern for the safety of customers" a Megabox official told AFP.

She said groups of conservative activists had threatened to stage rallies outside Megabox cinemas that screen the movie.

But Megabox declined to identify who the activists were.

Produced by firebrand director Chung Ji Young, the film explores alternative explanations about possible causes of the sinking, including a suggestion that the vessel might have run aground on a reef and then collided with an unidentified submarine.

Megabox's withdrawal of the film left it with only nine small cinemas for screening across the country.

"How could such a thing happen now? This might happen only under a military dictatorship", Chung told Yonhap.

"This is just outrageous".

Director Baek Seung Woo has said his documentary aimed to expose a South Korea where "even reasonable questions about government declarations are not permitted, where no communication happens".

Project Cheonan sparked an outrage from conservatives, the Defence Ministry and families of the dead sailors after it was first shown in April at the Jeonju International Film Festival.

Last month, a senior navy officer and the chairwoman of an association of surviving family members filed a request for courts to prevent it from hitting the screen, claiming that it "distorts the facts and defames the individuals involved".

But on Wednesday, a court in Goyang City near Seoul dismissed the request, citing the constitutional right to freedom of expression.

"Given that the film was made to argue a need for discussions on various allegations over the cause of the Cheonan's sinking, we cannot conclude that it stated falsehoods," the court said in a statement.