A reform of the law is needed to ensure a proper public interest defence for journalists, Nick Clegg has said.

The Liberal Democrat leader criticised the "opaque" nature of the current system and said prosecutors were relying on outdated laws in deciding whether journalists should go on trial.

Mr Clegg, speaking as he launched the Lib Dems' election campaign said he had been concerned about the way the law was being used for some time.

He said: "I have long been concerned that the laws of the land are not clear enough on the public interest defence for journalists and other people who are covering information in the interests of the public."

Mr Clegg added: "It's just far too opaque, in too many of our laws, exactly what is the strength and nature of a public interest defence.

"I would like to see that clarified in law, my party has always advocated that. The fact that prosecutors are relying on 13th century laws, that we don't have an up-to-date definition of what a public interest defence is, shows the need for a proper review and a proper reform of the law in this area."

His comments follow the quashing of the conviction of the first journalist to be found guilty of paying a public official in the wake of the high-profile Operation Elveden probe.

The former News of the World reporter was handed a six-month suspended sentence after being found guilty of paying a prison officer for information.

In turn, the officer was jailed for three-and-a-half years for misconduct in a public office while his friend was given 30 weeks behind bars.

All three launched an appeal their convictions were quashed.