It has been likened to the worst breach of parliamentary privilege since the mid-17th century but this time with the Speaker found wanting. The arrest of Damian Green, MP, the searching of his home and raiding of his Commons office have provoked almost universal and cross-party outrage.

On the eve of the Queen’s Speech this Wednesday, pressure is mounting on the Home Secretary from opposition MPs who have effectively accused her of knowing in advance about Mr Green’s arrest. Her apparent defence of the police has stoked their fury. Her denial was challenged by her shadow, Dominic Grieve, as “extremely unconvincing”.

Meanwhile, Dr David Green (no relation), the director of the thinktank Civitas, has called on Parliament to impeach Sir David Normington, Permanent Secretary at the Home Office, along with any other senior officials who called in the police to investigate leaks of information from Christopher Galley, a junior Home Office civil servant.

The police, too, are under fire for heavy handed tactics involved in arresting the MP, holding him for nine hours and ransacking his homes and offices.

The mix of the criminal law and Parliament is an explosive one. No one, though, is saying that MPs are above the law. The consensus this week was that their work enjoys certain privileges and any encroachments into their communications and place of work need to be justified to what Alan Beith, MP, chairman of the justice committee, called a “high threshold”.

“MPs,” he said, “have to be able to communicate with their constituents secure in the knowledge that matters will not be disclosed to the executive; or indeed that there will be an investigation . . . where the ability of an MP to do his work both as a constituency member and holding government to account may be impaired”.

Dawn Oliver, Emeritus Professor of Constitutional Law at University College London, agreed. “There is a very strong expectation that MPs are not to be harassed in carrying out their lawful and constitutional functions, and generally speaking constituents have to be able to communicate freely and in confidence with their MPs.”

There is a second issue: police might pick up information that was beyond the scope of any warrant issued, she said. These privileges are not enshrined in statute. It is a question, she added, of tradition and centuries of decisions by the House of Commons.

So what is the law governing MPs? The bible of parliamentary law, Erskine May’s Parliamentary Practice, makes clear that MPs enjoy the protection of privilege for proceedings in Parliament. Vernon Bogdanor, Professor of Government at Oxford University, said it does not extend to MPs’ offices — by which he meant that an MP accused of keeping stolen goods in his office would not be safe from a criminal or a police investigation. Yet great care must be exercised. Professor Oliver and others said that MPs could not carry out their work if they felt that their files and computers could be seized at any time. The raid on Mr Green’s office, she added, “might have come close to a contempt of Parliament”.

The second key issue is the law on disclosure of government information. Until the infamous and catch-all Official Secrets Act of 1911 was repealed, it was a criminal offence so much as to reveal the colour of the carpet in M15’s offices.

A series of highly publicised acquittals led to an overhaul of the Act, with the aim of ensuring that only certain categories of information — such as that bearing on security, intelligence, defence, international relations or that impedes the prevention or detection of offences — fell within its ambit. And the prosecution would have pass a specific test showing harm had been caused. In Parliament, Douglas Hurd, then Home Secretary, whose measure it was, said: “We have deliberately narrowed the scope of possible prosecution by providing, in most cases, a specific harm test.”

In future, he said, “it will not be possible for it to argue against a civil servant who reveals a defence secret that the disclosure damaged some vague public interest”.

The Official Secrets Act of 1989 would remove the great majority of information from the criminal law. So none of the information allegedly leaked by Mr Galley to Mr Green falls within its scope, according to Maurice Frankel, director of the Campaign for Freedom of Information.

Leaking it, therefore, did not appear to be a criminal offence; nor was the receipt of it, unless the information was procured in some way.

“If it was not criminal for the civil servant to hand over the information, it was not criminal for the MP (or a journalist for that matter) to receive it, unless there was money handed over, which does not seem to be the case,” he said.

If not a breach of the Official Secrets Act, it would nonetheless be a breach of confidence and contract for a civil servant to disclose confidential information, unless he had a public interest defence and had sought and been unable to raise matters of concern through the procedures set up for would-be whistleblowers. Such a breach of terms of employment could lead to dismissal.

So what is Mr Green (and Mr Galley) accused of? Police are investigating an obscure common law offence — conspiracy to commit misconduct in public life. The Director of Public Prosecutions has been kept informed and a couple of meetings were held between the police and a senior casework lawyer at the Crown Prosecution Service before Mr Green’s arrest.

The police have said that Mr Green is being questioned in connection with “aiding, abetting, counselling or procuring misconduct in public life”. So the issue is the nature of the relationship, if any between the MP and the civil servant. Was the latter induced or encouraged — perhaps in the hope of a later job at Conservative Central Office? These are the questions police are seeking to answer.

There is a wider issue. If Parliament legislated to remove most leaks of government information from beyond the ambit of the criminal law, what would it make of an attempt to use an obscure common law offence instead to catch such leaks? It could be said, one lawyer noted, that prosecution under the common law “amounted to a deliberate subversion of Parliament’s intention”.

Both men at the eye of the storm have instructed some of the best lawyers in the business: Mr Green has hired Michael Caplan, QC, with Stephen Parkinson and Eve Giles, all from Kingsley Napley; Mr Galley has Neil O’May, head of criminal law at Bindmans. The lawyers will have this argument about “subverting Parliament’s intentions” at the forefront of their armoury if any case gets even near a courtroom door.

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