The application of the obscure common law offence of aiding and abetting misconduct in a public office in Damian Green's case is highly unusual, according to legal experts.

Jeremy Summers, Partner and Business Crimes Specialist at law firm Russell Jones and Walker, said it is generally used in the context of corruption in public office - for example, for police officers improperly accessing the national police database.

It would be the Home Office official who had committed the alleged primary offence in leaking the information, he explained, adding: "This is the secondary level which they're seeking to attach to the shadow minister."

Damian Green was arrested for allegedly abetting an agreement to commit a criminal offence, essentially for "egging on" the Home Office official, Mr Summers added.

The offence carries a maximum sentence of life imprisonment and in principle could be applied to journalists or any third party seen as conspiring with or aiding and abetting a public official to leak information.

Journalist Sally Murrer learned this to her cost in May 2007, when she was arrested over tips she had received from Police Sergeant Mark Kerney and published in the Milton Keynes Citizen. Interrogated three times, strip-searched and held in custody for 24-hour periods, the 50-year-old Buckinghamshire mother-of-three was charged and told she would go to jail for her alleged crime.

Today, the cases against Ms Murrer and Mr Kearney collapsed, the judge having ruled that evidence from a massive police bugging operation was inadmissable as the surveillance was a violation of human rights. Crucially, he also concluded that British and EU law afforded protection to journalists and their sources, citing rulings in cases across the continent where police had used heavy-handed methods to try and prosecute reporters.

Mr Summers said the case against Mr Green was likely to meet the same obstacles. The Crown Prosecution Service will have to decide whether a prosection would have sufficient chance of success and whether it is in the public interest. It will also have to be satisfied that Mr Green wilfully committed the alleged offence - that he knowingly persuaded the official concerned to act improperly - and did so in away that constituted an abuse of office and a breach of the public trust.

Today both the Conservatives and Liberal Democrats insisted that Mr Green had revealed information in the public interest, arguing that this was a key part of any MP's job.

Professor Gary Slapper, Director of the Centre for Law at the Open University, said the application of this "nebulous" law to such leaks was a "diminishment of our democracy".

"Leaks are committed all the time often with the apparent complicity of government minsters and no criminal investigations follow. There was a rather shocking governmental leak of pre-budget information earlier this week. If leaking is suitable to become the subject of serious criminal investigations, it will have to apply equally to those from government as well as to those from Her Majesty’s opposition."

"When the state starts arresting elected parliamentarians for saying things it says they shouldn’t, alarm bells should ring," he added.

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