Can a journalist publish stories that might not be true?

What happened?
Michael Charman was a former detective constable in the Metropolitan Police force who claimed that he had been defamed in a book called "Bent Coppers" written by Graeme McLagan and published by Orion Books Ltd.

The book claimed that Charman, and his informant Geoffrey Brennan, fabricated stories about money laundering and gun running in order to conceal their own involvement in criminal activity. Brennan had also previously accused Charman of receiving payment in exchange for his protection. McLagan’s book detailed how Brennan had given Charman up to £50,000 to protect him while he carried out a substantial fraud. Police executed search warrants at Charman’s home, and he was suspended from duty in 1997.

Chapman argued that the book libelled him by suggesting there were "cogent grounds" for suspecting him of being involved in police corruption.

Judgement: be responsible!
The appeal court judges ruled in favour of Orion Publishing. They said that although the allegations about Mr Charman could not be proved, Mr McLagan, a former BBC home affairs correspondent, had taken steps to verify the story and that as a result of his honesty, his expertise, his research and his thorough evaluation of a mass of material, the book qualified as responsible journalism.

Essentially the High Court decided that if the subject matter was of public interest and the steps taken to gather and publish the information was responsible and fair than this constituted responsible journalism.
This ruling has interesting implications for investigative journalism because it accepts that even if a journalists’s claim cannot be proven, as long as the writing is ‘responsible’ then the journalist can publish their findings.