So now we have a leadership race. Of course, it has not been announced yet, but it seems an inevitability after today’s heavy defeat for Jeremy Corbyn in a confidence vote among Labour MPs.

Corbyn is down indeed, but not out. Despite being hit a tidal wave of resignations from the frontbench over the last three days he is hugely confident he can win another vote of Labour members and tonight issued a statement decrying the confidence motion as having “no constitutional legitimacy”.

In fact, Corbyn is defiant. Sources closer to the leader have repeatedly said he won’t be taken out by a “corridor coup” and told any challenger they are welcome to make a move as soon as they can get the necessary 51 signatures from MPs and MEPs. Or, to put it another way, bring it on.

But if a leadership election is a near certainty than the identity of the challenger remains a mystery. After nine months of agonising, the so-called moderates in the Parliamentary Labour Party (PLP) are still to settle on the one person who would have the best chance of beating Corbyn in a run-off.

You don’t need me to remind you of the possible names in the frame: Tom Watson and Angela Eagle have been considered as conciliatory figures from the soft left. Dan Jarvis, the former paratrooper, has been touted as someone with the personal and political courage to take on the left. Chuka Umunna is clever, youthful and articulate – and may want another crack at running after last summer’s false start. And then there is Yvette Cooper, third in the previous contest, but with significant Cabinet experience and now a track record of campaigning for action to tackle the refugee crisis.

None have vowed to stand and none have ruled themselves out. The betting is, increasingly, on Eagle. Clever, quick-witted, and increasingly impressive at the despatch box – as shown by her occasional appearances deputising for Corbyn at Prime Minister’s Questions – she is emerging as the favourite. Given her relatively low profile in Ed Miliband’s shadow Cabinet, then why? She would be acceptable to most of the moderates and would be thought to have a fair chance of winning votes from the left, given the nine months she spent serving in Corbyn’s top team as shadow Business Secretary. And she has experience as a minister in the Blair and Brown governments, and in a series of roles in the shadow Cabinet, as well as in the chair’s role at Labour’s National Policy Forum.

Whatever the outcomes of the tense negotiations now going on in the quiet corners of Portcullis House – where most Labour MPs are based – there can only be one candidate. The nightmare for the big beasts who have sat on the backbenches since September is of two moderates putting themselves forward and splitting the anti-Corbyn field, allowing the veteran left-winger to win easily, just as he did last time.

A two-horse race will bring some clarity after several days of the most public and painful uncertainty in the aftermath of the Brexit vote. But any real sense of resolution will only come after a leadership election, whatever the outcome.

The starting gun for that election was not fired today but it cannot be long before the race gets underway. And don’t assume anything other than that it will feature Corbyn on the ballot paper. Yes, some in the anti-Corbyn camp believe they could prevent him from being a candidate and feel confident in a legal opinion that the incumbent needs to gain the nominations of 20 per cent of MPs and MEPs to be able to stand again. But Corbynistas have their own legal advice saying the opposite and are not losing sleep over the prospect of their man being barred.

Corbyn’s supporters have already resumed their lobbying of MPs, just in case he were to need a fresh round of signatures to stand again but, interestingly, those same MPs have also reported they are receiving an influx of email from members frustrated at some of the stranger decisions of the leadership, whether on policy or on his willingness to appear on late night television shows.

The truth is the decision of whether to automatically grant Corbyn a place on the ballot paper will be a political rather than legal judgement – and it is impossible to see Labour’s national executive committee (NEC) denying Corbyn his intention to fight on.

So we are looking at a leadership contest. While some longstanding centrist members and MPs will be joyful at the chance to “reclaim” the party, the left will double down confident in the knowledge they won the last battle decisively less than a year ago. Others will simply sigh at the prospect of weeks or even months of internal debate at a time when the Tories are in turmoil.

Wherever you stand, it will be a battle for the soul of the party. And it looks like it could start very, very soon.