Paul Linford used to be a fine lobby correspondent and he now writes one of the most thoughtful political blogs. Ever the shrewdy, he has an interesting take on John Reid's departure:

As ever with Reid, there are quite a few theories, and they can be summarised thus.

1. He is genuine. He is coming up to 60, wants to take a break from government, and wants Gordon Brown to have the freedom to bring in his own people as he said yesterday.

Probability rating: 2/10. Reid is a politician to his fingertips, and it just doesn't square with what he said last autumn.

2. With the forthcoming break-up of the Home Office, Reid's role is about to diminish and Gordon was unable to offer him anything bigger by way of compensation. There is some speculation that he might have asked for a combined Defence and Homeland Security brief

Probability rating: 6/10. Gordon would have been happy to keep Reid in Cabinet in one of the two Home Office briefs, but not in a beefed-up role.

3. He has been forced out by some impending tabloid scandal. This is the theory currently running on Iain Dale.

Probability rating: 4/10. Reid has a fairly colourful past but it's unclear to me whny him resigning would make a tabloid newspaper any less likely to print something.

4. He is staging a canny tactical retreat to distance himself from what he sees as the impending disaster of the Brown premiership so that he can live to fight another day after the next election.

Probability rating: 7/10. There is no love lost between Reid and Brown and his decision not to serve could be likened to Iain Macleod's under Douglas-Home in 1963.

My conclusion, then, is that this is an act of deep disloyalty on the part of Reid which will weaken Brown and weaken Labour in the run-up to the next election.