Sir Paul Stephenson's decision to review the police handling of the Damian Green investigation puts the Commons Speaker, Michael Martin, in an even more awkward position as he prepares his statement defending the action of the Commons authorities in allowing the raid on the immigration spokesman's office.

Conservative MPs are happily drawing the inference that the announcement of a review is an acknowledgement that things might have gone wrong here.

And if the police themselves are concerned enough about it to review an investigation that is only just under way, it means that the questioning of the Speaker's role is likely to be all the more trenchant.

The Conservatives will be hoping this is the first step to calling off the investigation. Labour will be in two minds. Many MPs and ministers are deeply embarrassed about the affair and want it to be put to bed as quickly as possible.

Others believe that the Conservatives have created an almighty fuss about the constitutional proprieties of the case to deflect attention fromthe fact that one of their number has been arrested and has been questioned about whether he behaved improperly. They believe that if the investigation is allowed to run its course, more might be be revealed.

Whatever happens in the future, tomorrow's State Opening and Queen's Speech will be overshadowed by a row that has run out of control.

Mr Martin knows that he needs to draw the sting and will almost certainly announce his own review of the procedures to be followed by the Commons on the rare occasions it receives requests from the police to interview MPs.

What he says will determine whether the Conservatives drop their offensive or pursue it in tomorrow's debate in which David Cameron and Gordon Brown are the star turns.

The affair has put Jacqui Smith, the Home Secretary, under pressure with a string of questions about what she did and did not know.

Today she welcomed the police review and revealed that she sought assurances from Sir Paul yesterday that the inquiry was being carried out in a “proportionate” manner.

The change of tone was noticeable. On Sunday she had refused to give any kind of hint that she felt the police behaviour might have been excessive. Now it is over to the Speaker.

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