Sadiq Kahn, the mayor of London, has become the latest senior Labour figure to declare that Jeremy Corbyn has to be stopped if the party is not to be routed at the next general election. Pragmatic and street-smart, Khan is increasingly emerging as a possible de facto leader of the moderate wing of Labour. “I am afraid we simply cannot afford to go on like this,” he wrote.

But the sad truth is that Labour will keep going on like this. This is the party’s worst crisis since it won only 52 seats in the 1931 general election after Ramsay MacDonald had broken away to form the National government. After losing the 2015 election under the feeble leadership of Ed Miliband, the party has been captured by the far Left and ceased to be in any recognisable sense a government in waiting. And the gulf that has opened between its Left and its mainstream may never be bridged.

Corbyn is likely to be re-elected as leader at a special conference on September 24, cheered on by hundreds of thousands of activists who aspire to turn Labour into a mass-membership social movement irrespective of its election-winning potential. For these people, many of them young idealists, there is no turning back to the triangulations and equivocations of recent years. They believe policy should be made by the popular will of the many, not by the elected few. They are ashamed of and despise Tony Blair, and they will never allow Labour’s most electorally successful leader and those who supported him to forget the catastrophe of the Iraq war. They believe that moderate social democracy has failed. They wish to make no accommodation with the capitalist order; comical as it may sound in 2016, they wish to overturn it and create a new economic paradigm.

With Corbyn re-elected, the Left will continue with its transformation of Labour, taking control of its policy-making committees and constituency parties as well as agitating to have “moderate” MPs deselected. In return, the MPs say they will form what may in effect become a party within a party, meeting and organising together as they seek to prevent Corbyn from turning Labour into little more than a rainbow coalition of disaffection and protest.

If you speak to Labour MPs, they are despairing but not completely resigned. There have been discussions about a split and the creation of a new pro-EU, pro-business centre-Left progressive party. This is wishful thinking. Who, for a start, would lead it? There is no one of the stature of Roy Jenkins, Shirley Williams or Denis Healey. There are gifted MPs – Dan Jarvis, Keir Starmer, Rachel Reeves, Chuka Umunna – but they do not yet have a record of hardened achievement. Nor do they wish to quit and start again in a new party.

Joe Haines, the former journalist and aide to Harold Wilson, has a different plan. Describing Labour as being “unelectable but not indestructible”, he advocates MPs forming a breakaway parliamentary faction – perhaps as the Peelites did after the repeal of the Corn Laws in the 1840s. On the strength of numbers alone, this new faction could declare itself the official opposition, in effect splitting the PLP from the leader.

Yet Labour has already split. “It’s like one of those chasms you see in the Arctic,” said Peter Kyle, the MP for Hove and Portslade, of his party in the New Statesman last week. “It starts very small at the top, a dusting of snow covers it; but underneath is this enormous gap, and when somebody steps on it you fall through.”

What sustains Labour MPs even in their darkest periods, when elected office seems like a chimera, is a sense that their values are superior. At the launch of her ill-fated leadership campaign in July, Angela Eagle was asked how she would beat Theresa May. Her reply was as pithy as it was revealing: “Because she’s Tory.” There was nothing more sophisticated to be said. Her complacency was damning but characteristic of much Left-wing thinking. Too many Labour people think that the Tories are essentially nefarious and so cannot understand what is so attractive to so many about conservatism.

For his part, John McDonnell, the neo-Marxist shadow chancellor, recently claimed to come from the mainstream of the Labour tradition, citing Clement Attlee as an influence. McDonnell is more intelligent and better read than Corbyn, but in this instance he was being deliberately obtuse. Attlee loved his country and its institutions. He was formed by Haileybury College, the poetry of Kipling, and his experiences as a soldier in the Great War. He became a socialist after working among the poor of the East End at Toynbee Hall. And he believed in strong defence and collective security: Attlee’s Labour was the party of Britain’s nuclear bomb and Nato. These aren’t the politics of Corbynism, which is pacifist, isolationist and anti-patriotic.

In his forthcoming Attlee biography, Citizen Clem, the brilliant young historian John Bew urges Labour to recapture something of the ethos of the Attlee period – of what he calls Attlee’s “unobtrusive progressive patriotism”. It may be too late. “I started writing the book four years ago in the hope/expectation that a revival of Attleeism was Labour’s last great hope,” Bew told me. “Now I’ve come to the conclusion that it’s almost a post-mortem.”

This is not yet the end of days for Labour, but the party is in a critical condition. Its problems go far deeper than Corbyn’s leadership; he is a symptom rather than the cause of the malaise. More broadly, social democratic parties are being rejected across Europe, their purpose weakened by globalisation and immigration, and overwhelmed by the populism of the radical Left and Right.

The Brexit vote was a further warning to Labour of just how disillusioned its traditional supporters are. And it could be that the politician best placed to appeal to them is Mrs May. For her steely resolve, cool mind, professed concern for social justice, and her understanding that a strong state is the precondition of order and security seem more in keeping with the spirit of this anxious age than the Labour leader’s peacenik utopianism.

Jason Cowley is editor of the New Statesman