Will this be the end of the affair?

Labour's cynical decision to vote against the AV referendum could potentially spell the end of the coalition.

The Bill will come before the House of Commons to vote in the autumn and Labour MPs will be whipped to oppose a measure they themselves had in their manifesto. Their official reasoning is that the Bill will also include plans to reduce the number of MPs. But I believe that their real motive is that they want to cause as much friction as possible in the coalition, perhaps even to break it apart.

Of course, Labour on their own cannot defeat the Government. But there are also 44 MPs, mainly Conservatives who have backed a Commons motion opposing the timing of the AV referendum. Some of these will follow the three line whip imposed on Government MPs on this issue and others will vote in favour of amendment on the referendum timing but back down when it comes to the core principle. There may also be a few Labour MPs who think it is wrong to vote against a plan they campaigned on just three months ago. But it is still a dicey moment for Cameron and Clegg.

If the AV Bill fails then I cannot see any hope for the coalition to remain. Staying in Government with the Conservatives in such circumstances would certainly split the Lib Dems, many of whom would see the desire for bums around the Cabinet table as being less important than remaining true to a fundamental (perhaps the fundamental) policy of the party. AV is not the electoral reform that most Lib Dems want, but it is emblematic of the relationship and no amount of persuasion will convince us otherwise.

Come the autumn, it will be the turn of the whips to take centre stage. But before that there will be a long hard summer for Nick Clegg and David Cameron as they try to keep the coalition together.