Two good reasons for PM not to fire Vince Cable

Why doesn't David Cameron simply fire Vince Cable? The business secretary has stepped way over the mark in calling the Prime Minister's speech on immigration "very unwise" and, according to the Sunday Times, he's gone further.

The paper claims Cable has advised the Association of UK Private Schools and Colleges to mount a legal challenge to stop the UK Border Agency withdrawing licences to admit non-EU students.

Cable insists he did not directly encourage the colleges to go to court but Neil Mackay, principal of Alfred the Great College in London, told the Sunday Times: "Dr Cable has been brilliant. He advised us to take out an injunction. He was obviously shocked and appalled by the effect of this policy on our business."

Matthew D'Ancona, writing in the Sunday Telegraph, argues very reasonably that Cable should have been sacked for breaking the coalition cabinet rules which state clearly that all government ministers are bound by collective decisions - "save where it is explicitly set aside" as with AV. (Which is why Cable can share a pro-AV platform with Ed Miliband today with impunity, and Cameron can join Labour heavyweight John Reid at an anti-AV rally.)

With Libya and the NHS crowding the PM's agenda, and with the May 5 elections and AV referendum fast approaching, Cameron could be forgiven for wanting one fewer headache, said D'Ancona.

So, why hasn't Cameron fired him?

Two good reasons:

1. Why make a martyr out of an already diminished figure? Cable has been in a weak position ever since he was snared by the Daily Telegraph honey-pot before Christmas, when he boasted to two female reporters masquerading as constituents that he could bring the government down whenever he wished. (He also bragged, of course, that he was "at war" with Rupert Murdoch.)

2. Chancellor George Osborne is running rings around Cable on banking issues. Until Cable makes problems on his own patch, he's better ignored. He's less dangerous inside the tent than out, where he could become a real nuisance.

That said, you couldn't blame Cameron for saying enough is enough on the Cable front.

However, if and when that time comes, he cannot fire Cable without Nick Clegg's say-so (indeed, it will doubtless by Clegg who does the firing). However irritating the Lib Dems are becoming, Cameron and Clegg's futures are intertwined. They are joined at the hip, as a Tory spin doctor put it to the Mole last night.

Or as Cameron himself put it on Sky TV at the weekend, "Whatever the result on May 5 [and, he could have added, whatever Vince Cable might wish], this is a five-year government... Nick and I are absolutely committed to taking the government and its programme forward." ·