A man confessed to a journalist he was involved in the 1974 British pub bombings that killed 21, a court heard Friday, in a case brought by police trying to force the reporter to reveal his identity.

West Midlands Police wants Chris Mullin, 74, to disclose source material from an investigation he carried out between 1985 and 1986 but he is challenging the application.

London's Old Bailey court heard that during the course of the probe, a man made a "full confession" to Mullin.

Six men were initially convicted of the attacks carried out on two pubs in Birmingham, central England, on November 21, 1974, which were blamed on the Irish Republican Army (IRA).

But Mullin's investigation helped clear the men and expose one of Britain's most notorious miscarriages of justice, leading to the release of the "Birmingham Six" after their convictions were quashed in 1991.

West Midlands Police are now using anti-terrorism legislation to try and force Mullin to hand over tapes and notes of the interview.

Police lawyer James Lewis told court that Mullin -- a former junior foreign minister in prime minister Tony Blair's government -- refuses to identify the bomber.

"Mr Mullin refuses to identify him because he says he promised (bomber) AB he would not reveal his identity," he told the court.

Lewis said the interview contained "a full confession to the murders".

Mullin was expected to argue that disclosure would breach the principle that journalists should protect their sources. He is being supported in the case by the National Union of Journalists.

"If West Midlands Police had carried out a proper investigation after the bombings, instead of framing the first half-dozen people unlucky enough to fall into their hands, they might have caught the real perpetrators in the first place," said Mullin in January.

"It is beyond irony. They appear to have gone for the guy who blew the whistle."