The NUJ believes that videotapes which show prisoners being interrogated in Iran reveal a conspiracy to conceal the truth about the murder of journalists.
The tapes, in CD form, were passed to the union by exiled opponents of the Iranian regime.
They show the victims of brutal interrogations being threatened and beaten until they admit various crimes, including working as agents for the US and Israel.
The Iranian authorities have always claimed "rogue elements" in the security services were behind a spate of murders of journalists and writers. Five were killed between November 1998 and February 1999.
But secret trials of those alleged to have carried out the murders have been dismissed by the victims’ families and human rights and journalists organisations as a sham designed to appease international opinion.
They believe the trials were an attempt to protect senior figures in the Iranian regime.
NUJ general secretary John Foster, speaking at a press conference revealing the contents of the tapes, said: "The tapes are very important and we think indicate a conspiracy to conceal the truth about the serial killings of journalists and writers in Iran.
"We have been very concerned over the past few years about the murder of journalists by death squads. We know that the Ministry of Information has argued that the murders were nothing to do with them and that it was rogue employees who were agents of the Israeli or US governments. We are sceptical about that."
Foster warned that the union could not be sure that the tapes were genuine as the Iran regime was "full of dirty tricks".
But he said the NUJ wanted people to be made aware of them and to prompt a debate with MPs and the Foreign Office. The union is calling for an independent international inquiry into the murders.
lA transcript of one of the tapes quotes an interrogator saying: "In order to close this dossier and stop public attention [regarding the killing of journalists] we are supposed to release a number of names, claiming them as culprits, and even announce that these people were already killed."
By Ruth Addicott
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