The resignation of David Cairns brings to a close a short but classic Westminster manhunt.

It began late last night when the BBC said that it had spoken to a minister “just below Cabinet level” who was ready to quit.

That gave journalists - and Number 10 - a field of around 30 Ministers of State, the second most senior rank, on which to conduct the search. Mysteriously, Mr Cairns’ name emerged fairly early on. Was it briefed out by Number 10? It is, after all, in Downing Street’s interests that the irretrievably disaffected are flushed out before Labour’s conference starts on Sunday.

Certainly Mr Cairns’ friendship with Siobhan McDonagh - sacked as a whip last Friday for seeking to trigger a leadership contest - made him a prime suspect from the start.

It is likely that expressions of loyalty were sought overnight by Geoff Hoon, the Chief Whip, or others acting on his behalf. Once they were not forthcoming, a decision to deploy damage-limitation measures would have been taken.

But with the press pack in full cry, other - blameless - ministers got caught in the cross-fire. Jim Murphy, the Minister for Europe, was forced to make clear that he was not the BBC’s source while even the housing minister Caroline Flint was briefly touted as a stalking horse.

The resignation of a Scotland Office minister will not bring down Gordon Brown. However, its manner will do nothing to counter the impression of an increasingly desperate and chaotic administration.

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