Does the arrest of Damian Green mean that we are heading for a police state? Was he being arrested merely for doing his job as an MP?

Article 9 of the 1689 Bill of Rights declares that, "the freedom of speech and debates or proceedings ought not to be impeached or questioned in any court or place out of parliament".

Privilege, however, extends only to proceedings in parliament, and to the correspondence between MPs and constituents. It does not extend to other activities that MPs undertake in the course of doing their job.

Further, as Erskine May, the bible of parliamentary procedure declares, "Not everything that is said or done within the precincts forms part of proceedings in parliament". A casual conversation in the corridors of Westminster that revealed an official secret would not be protected by privilege.

Parliamentary privilege has never extended to freedom from criminal arrest. In 1939, when Captain Ramsay, MP, was detained under wartime defence regulations it was ruled that there had been no breach of privilege. It would be very odd if, simply because someone was an MP, this were allowed to interfere with the administration of criminal justice. There would then be one law for MPs and another law for the rest of us.

Some have alleged that the home secretary was using the police to silence a political opponent; others inconsistently allege that she should have intervened to stop the arrest. The police, however, are operationally independent, and the less they have to do with politicians the better. If the police did inform the home secretary before the arrest – and it is not yet clear that they did – Jacqui Smith would simply have replied, "Thank you for informing me". Anything more would have been unconstitutional.

The police followed the correct procedure in asking the Speaker, Michael Martin, if they could search Damian Green's office. Had the Speaker refused, he could have been accused of condoning suspected wrongdoing. An MP's office cannot be out of bounds for the police. Suppose that an MP had committed a theft and had hidden stolen goods in his office. Would it seriously be maintained that the police should not have a right to search that office?

If, therefore, the police had genuine reason to believe that Damian Green had been involved in a serious criminal offence, they had a duty to act as they would have done with any other member of the public who had fallen under suspicion. Of course, the police are themselves as accountable for their actions as other public servants.

The Metropolitan police are accountable both to the home secretary and to Boris Johnson's Metropolitan Police Authority. They will, no doubt, be called upon to explain and justify their actions. But it is absolutely vital to maintain the principle that, apart from proceedings in parliament, MPs are entitled to no special privileges, that they cannot be a caste apart. That, after all, is what constitutional democracy means.