When Henry II bellowed "Who will rid me of this turbulent priest?" and four of his knights journeyed to Kent to murder Thomas a Becket, he famously showed abject contrition by walking to Canterbury in sackcloth and ashes.

When Jacqui Smith's petty tantrums at leaks from the Home office set in chain a series of events that ended in a posse of Counter Terrorism officers journeying to Kent to arrest Damian Green, we saw no such contrition. Her responses went from "I didn't know," to "It's not my responsibility," to "It would have been irresponsible not to have done it." That last claim was based on the assertion that national security was at risk.

Let us put to one side the Home Secretary's inability to distinguish the nation's security from her own political insecurity, and consider the facts from a man who is both independent and has all the facts, the Director of Public Prosecutions. He said, " I have concluded that the information leaked was not secret information or information affecting national security.

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"It did not relate to military, policing or intelligence matters. It did not expose anyone to a risk of injury or death. Nor, in many respects, was it highly confidential. Much of it was known to others outside the civil service, for example, in the security industry or the Labour Party or Parliament. Moreover, some of the information leaked undoubtedly touched on matters of legitimate public interest, which were reported in the press."

Throughout this case Ministers have tried to imply that there was some hidden sinister aspect to this case. Indeed, the letter to the police from the Cabinet Office said in terms that there was a threat to national security. We now know that this was untrue, that it was either a lie or a piece of remarkable incompetence.

So how did it happen?

What the combination of the Damian McBride affair and the Damian Green affair tells us is that a culture of vindictiveness has grown up, in Whitehall in general and the Cabinet Office in particular. It is a culture that can only have originated from ministers and special advisers, since it is so at odds with the traditions of our Civil Service.

Indeed, the increasingly habitual use of the full power of the state to attack opponents of the government, be they whistleblowers or political rivals, is inimical to our entire liberal democratic tradition.

There are a number of questions left unresolved. Firstly we have not established whether Ministers were knowledgeable about these decisions and actions; and if they were not, why not? Police operational independence does not absolve Ministers from accountability. Are we seriously to believe that in the six month gestation of this case, no minister was consulted at any time?

Former Home Secretaries of both parties have said to me they are astonished that Jacqui Smith didn't know what was happening. She should have done. We also still have the unresolved question of Parliamentary privilege, which was undoubtedly breached in pursuit of this case.

It is the responsibility of Parliament and a free press to hold Government to account. When Governments lie or conceal information, as this one has done systematically, it is necessary to publish information from whistleblowers. They will not be possible if the police can seize parliamentary files whenever they feel like it.

Parliament also needs to consider the question of when criminal law can be used to protect confidential government information. If an employee leaks confidential information when working for a private company or individual - a celebrity's nanny, for example - they get the sack. They do not get arrested.

Government is only different in very limited circumstances, essentially involving national security. Those limited circumstances do not extend to protecting Labour politicians from embarrassment and exposure of their failures.

That is why the Damian Green affair is the second major case attempting to use "misconduct in public office" to gag whistleblowers to collapse in twelve months.

This law is antiquated, anachronistic, and has become a discretionary power for the Executive rather than a reliable protection for the people. It is time it was revoked or rewritten. After all, if we took the law literally, namely "misconduct in public office", this week we might end up arresting the Home Secretary.

David Davis is Conservative MP for Haltemprice and Howden and a former shadow home secretary