When Labour fought the general election in May it had around 200,000 full members. It now has more than 380,000, close to the 400,000 reached in 1997 and an increase of around 90,000 since Jeremy Corbyn became leader.

But what is rarely noted is that these are net figures: the number who have joined minus the number who have left. Two months after Corbyn's election, MPs are struck by those exiting as well as those entering. Peep Show actor Robert Webb, who endorsed Yvette Cooper in the Labour leadership election, revealed yesterday that he had left the party - and he is far from the only one. One senior MP estimated that for every 75 members who joined, around 25 left. The majority of those departing are likely, as in Webb's case, to have voted fo non-Corbyn candidates.

The net result is that the party has moved leftwards since the Labour leadership election. Were Corbyn to be challenged and defend his position, many believe he would win an even larger victory. As both supporters and opponents of the Labour leader emphasise, the increasing discontent among MPs is not shared by the membership.

For this reason, as I write in my column this week, the question of deselection, be it of Corbyn or of recalcitrant MPs, will persist. True unity will not be achieved until the PLP reflects the leader, or the leader reflects the PLP. As Labour's divisions grow, MPs fear that left-wing members will increasingly turn on them. One told me that Corbyn's refusal to grant a free vote on air strikes in Syria was "an attempt to pick a fight, isolate colleagues and pave the way for deselection". The list of the 21 MPs who abstained on George Osborne's fiscal charter, while the leadership voted against, was widely shared by Corbyn supporters. At a recent meeting of Momentum in Nottingham, former shadow chancellor and local MP Chris Leslie was mentioned as a target for deselection.

A more left-wing membership will make it easier for Corbyn to win conference policy votes and for his supporters to become parliamentary candidates. As the Labour leader's opponents privately acknowlege, it will also make it far harder for any candidate significantly to his right to win in any future contest.