The arrest of Damian Green last Thursday, his subsequent detention and interrogation, together with the police search of his home and his office in the Palace of Westminster, constitute the most serious breach of the privilege of Parliament in modern times. At least eight senior figures in the British Establishment were involved; they either initiated the action, agreed to it, conducted it, or allowed it to continue. Not one of them seems to have understood how serious a “high crime or misdemeanour” they were conspiring to commit.

The police may have thought that they were legitimately investigating a crime; in fact, they were committing one, a much more serious crime than the one they imagined they were investigating. Contempt of the House of Commons can only be defined by the House of Commons itself, but there is little doubt that this was it. All the evidence of history is that Parliament has to protect itself against outside pressure of all kinds, and particularly against coercion by the executive power.

In 1523, Sir Thomas More, as Speaker, had to resist the pressure of Henry VIII's Minister, Cardinal Wolsey; in 1642, Speaker Lenthall frustrated Charles I's attempt to arrest the five Members. The House of Commons needs the protection of privilege to do its job. The liberty of Members is the liberty of the people.

In the present case, had the police waited for 24 hours, they would have learnt of the acquittal of a journalist on the very charge they were investigating. Sally Murrer and her police source, Detective Sergeant Mark Kearney, were both acquitted under article 10 of the European Convention on Human Rights, which protects the right of every citizen to receive and impart information without interference by public authority.

It is not clear what legal advice was taken by the police before they decided to arrest Damian Green. Plainly it was inadequate. The leader of the House, Harriet Harman, has said that they did not consult the Attorney-General or the Solicitor-General; they certainly did not consult her.

Who played the role of Cardinal Wolsey, and had the arrogance to invade the rights of Parliament? If Jacqui Smith, the Home Secretary, is to be believed - and her testimony is not entirely convincing - two senior civil servants ordered the police investigation - the Cabinet Secretary, Sir Gus O'Donnell, and the Permanent Secretary at the Home Office, Sir David Normington. These civil servants have many questions to answer. What legal advice did they take? When did they consult their own ministers, which would be the Prime Minister in the case of Sir Gus O'Donnell? What steps did they take to supervise so sensitive a police inquiry?

Did they know the police were going to arrest a Member of Parliament? Did they consider whether that might be a contempt of the House of Commons? Did they consider whether a search of Mr Green's office in Parliament would be another contempt? I doubt whether, as Jacqui Smith suggests, these civil servants acted entirely on their own; I think it more likely that they had already discussed the alleged leaks with the Prime Minister and the Home Secretary before they asked the police to intervene.

The higher responsibility belongs to the Prime Minister and Home Secretary. We do not yet know exactly when Gordon Brown or Jacqui Smith knew that the actual arrest had taken place. It seems that Boris Johnson, as Mayor of London, was told before the event; he made a very proper protest. The Speaker and the Leader of the opposition were also informed before the arrest happened. It is quite hard to believe the two ministers were not told at the same time.

What was Gordon Brown supposed to say if he had not been informed but David Cameron or Speaker Martin decided to telephone him and ask him to call off this illegal event? Could Sir Gus O'Donnell have waited to tell Gordon Brown until after the event, when so many other people knew before the event? The story being told does not really hang together.

In any case, Gordon Brown and Jacqui Smith admit that they knew what had happened shortly after the arrest itself. At that time the contempt of Parliament was being continued and extended. Searches were being made, files and laptops were being removed. This essential contempt of Parliament could still have been mitigated by a telephone call from the Prime Minister or the Home Secretary.

The Speaker, Michael Martin, and the Serjeant at Arms, Jill Pay, failed to prevent the police invasion of the Palace of Westminster and may even have approved it. This breaks 700 years of parliamentary tradition. Both of them had the authority to keep the police out of Mr Green's office.

There were two senior policemen who must share part of the blame; it is no excuse that they were obeying orders. The junior of the two, Bob Quick, is an expert on terrorism. He will have to answer for the detailed handling of the operation. The senior was Sir Paul Stephenson, who was widely expected to succeed Sir Ian Blair as Commissioner of the Metropolitan Police; that would not now be appropriate or even tolerable.

The damage is strewn all around. Democracy is damaged; the House of Commons is damaged; British liberty is under attack. The police are damaged. The actual operation was a spectacular public disaster; many people now think we are living in a police state. The Government has been damaged. They must now realise how angry the public are. The Labour party has been weakened; this has been a horrifying mixture of ignorance, incompetence and shame.

Of course, Jacqui Smith should resign like other Labour home secretaries. She has been responsible for a major political disaster. I do not expect that Gordon Brown will resign, unless it proves that he did indeed have prior knowledge. Yet he bears the ultimate responsibility. The House of Commons remains the centre of our democracy. The freedom of Britain cannot survive if the centre does not hold.

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