Justin Davenport: Vulnerable Met seem to be desperate

Senior officers at Scotland Yard are said to have been taken by surprise by the extent of the controversy surrounding the arrest.

The review of the case may give them an exit strategy - or it could confirm they have got it right.

But the wonder is that they were surprised at all. The Met has taken on some of the most difficult police inquiries in recent years - the cash for honours and the Royal butler case are just two.

A decision to arrest a senior MP and search offices at the House of Commons must have rung alarm bells. As one chief constable told me: "You have to be damned sure of your evidence before you go around arresting MPs."

The timing of the case might explain the difficulties. The original request to investigate was made in the second week in October, just after Sir Ian Blair was ousted by the Mayor and before his deputy Sir Paul Stephenson had taken charge. There was a leadership vacuum.

Sir Ian was first told of the case and then passed it on to Sir Paul. Both may have taken their eye off the ball. There are some police chiefs who say they should have refused to investigate, bouncing the matter back to the Home Office.

But Met insiders argue they had little choice with concerns over the ability of the Government to operate.

But the inquiry again exposes the difficulties the police find themselves in today. Tony Travers of the London School of Economics points out the days are over when the authority of the law went unchallenged.

Unlike America where police chiefs can be defended by political masters, today's Yard bosses are exposed to criticism from all sides.