It must rank as one of the best-kept political secrets - and one springing from that most homely of Commons relationships, the "pairing" arrangements that offers MPs of opposing parties the chance to miss votes by cancelling each other out.
Margaret Hodge, MP for Barking and Tony Blair loyalist, could not have wished for a more helpful "pair" than Alan Howarth after she won the Barking by-election last year. According to Labour insiders, the formerly left- of-centre leader of Islington council quickly divined over lunch that he was further to the left than many in her own party.
Ms Hodge listened and observed Mr Howarth's growing disenchantment; they talked regularly. A few months ago there was another lunch, at which the possibility of a defection crystallised.
A long conversation between Mr Blair and Mr Howarth a week before Labour's Brighton conference was the clinching factor, along with the speeches delivered by Mr Blair and other front-benchers.Reuse content