Labour will lose the next general election if Jeremy Corbyn is still its leader, and lose it by a substantial margin. A distrusted and unloved Conservative Party will win something resembling a landslide victory. No ifs or buts, as David Cameron might say: that is the plain, unpalatable truth. Either he goes or the party itself is a goner. Those who believe otherwise are the Flat Earthers of British politics.

Barring a cataclysmic economic failure or a sexual scandal of unimaginable proportions, Cameron’s successor will have a shoo-in and a near-moribund Liberal Democrat party will get a kiss of life and dream of beating Labour for second place. Scotland, if it is still in the Union by 2020, wouldn’t offer a hope of returning to the Labour fold; the Scots voters know a wee, sleekit, cow’rin’, tim’rous beastie when they see one and Corbyn would be lucky to keep our only seat. Either we wake up or we shut up. Too bleak? Too pessimistic? A fantasy scenario and not realistic? No.

Things could get worse. The national party could, according to the gloomiest forecast, disappear altogether by next October, reduced to a municipal party with brave bands of councillors flying the flag, rather like the Independent Labour Party in the 1930s. Actually, I don’t think things are quite that bad. We face disintegration rather than immediate doom. At least at the first stage. But what will follow is a series of electoral defeats that can only be imagined.

It isn’t just the policies that Corbyn appears to represent, though these are little more than the outmoded slogans of the statist era, updated occasionally to appeal to youthful tweeters. The fact is that Corbyn, whatever views he holds, is not up to the job. I heard some of his first speeches when he entered the House of Commons in the early 1980s. They were empty then and they are empty now. He is not a Bennite, because Tony Benn would never have been so rigid in his thinking; his speeches were beautifully constructed, even when they were idiotic, and they did not consist of treadmill recyclings of Marx and Trotsky.

Corbyn has no vision for the future of Britain. He offers no beacon to light the way. Politically, he has the candlepower of a glow-worm. He might once have fitted the role of a deputy manager of a northern friendly society, kind, polite and compassionate yet unable to help his client, but he is intellectually unsuited to be a minister of any kind, let alone a prime minister.

But there’s nothing we can do about him, wail the Micawberites, until something turns up. That’s what they said about Gordon Brown – but nothing did turn up, except Ed Miliband. Chances were missed, not least by Ed’s brother, David, to supplant Brown. Those who knew the election might be lost hesitated to do anything about it and the election was lost. And the next one.

Look at Jeremy’s majority among the grass-roots members, say the Micawberites. Overthrow him and he will stand again and win again and we would be worse off than ever. Well, you can’t be worse off than dead. But let’s look at that majority.

According to the latest official figures from the party’s general secretary, Iain McNicol, Labour has 380,000 members, excluding the £3 supporters who were recruited by the trade unions, the Green Party, the mischievous Daily Telegraph, which encouraged Tories to buy a vote, and the odds and sods of hard-left Trotskyist, Maoist and Leninist groups who were expelled from the party or who left in disgust, like Arthur Scargill.

First of all, despite the common belief, Corbyn did not win a majority among the full members who voted. From them, he received 121,751 out of 245,520. The bulk of his vote came from “registered supporters” (the £3-ers listed above, 88,449 of whom voted for him) and the “affiliated supporters” – trade unions and a host of socialist societies ranging from the Fabians to Chinese for Labour – 41,217 of whom backed Corbyn.

The registered and affiliated supporters for Corbyn far outnumbered those for the rest of the candidates put together. I accept that even among members he would have won on the second ballot. But there is nothing unusual about that. Year after year, back in Harold Wilson’s time and beyond, the full members returned left-wing candidates to the National Executive, ranging from the dissolute Tom Driberg – whose publicly proclaimed left-leaning views blanketed the excesses in his private life – to “useful idiots” such as Ian Mikardo and Frank Allaun. They were always out of step with the parliamentary leadership, even Aneurin Bevan in his last years.

But let’s deal with reality: Corbyn’s total vote was just over 251,000; in other words, approximately one in every 183 people on the electoral register (46 million) voted for him, or 0.5 per cent. In relation to the next general election, that is the only statistic that matters and it should be compared to the nearly 9.35 million who voted Labour last May. The strength of the party lies in the nine million-odd, not the 251,000, and that figure will be dissipated at our peril.

It is the Parliamentary Labour Party that represents the Labour vote in Britain, not the 423,000 people, including the ragbag of “registered supporters”, who voted in the leadership contest. And it is up to the PLP to do something about it. Theirs is the true legitimacy. The parliamentary party is the most powerful force in the labour movement, far stronger than the total union membership, a significant part of which doesn’t vote for us anyway.

Many Labour MPs fear that Corbyn’s acolytes may organise their deselection. That is possible, though experience shows that deselections are few and far between. That threat could be removed if the choice of reselection was open to the whole membership of the local party, with 50 per cent required to support it. That is logical and democratic. Yet the bigger threat to them is not deselection by constituency activists, but de-election by the constituency voters. And that is going to be the fate of many unless the party implements a radical change of course. So what should it do?

Remember, the PLP cannot be dictated to within the party by any outside body. If the MPs decide they want to elect their own leader of the PLP they can do so. Jeremy Corbyn would be entitled to stand, though he might think it wiser not to do so, recalling that he would not have got on to the ballot in September, had it been not for the fallacy of “fair play” embraced by the likes of Margaret Beckett and Frank Field. Notoriously, Beckett describes herself as a “moron” for doing so. Corbyn may claim that his is the true legitimacy, though the statistics are against him. In any case, such a move would only be reverting to old practice.

The consequences for a new leader of the PLP would be difficult. He could appoint his own shadow cabinet but would he and they be recognised as such by the Speaker? What would happen to the Short money that finances the opposition? Who would sit on the front bench if Corbyn decided to hang on? Yet if there is the will, there is a way. He will argue strongly that the PLP is splitting the party, but if the majority of Labour MPs who voted against him in the leadership stood together, it would be he and his loyalists who would be the splitters. His 251,000 would not stand a chance against the representatives of the 9.35 million.

It would be wiser, if such an election were held, for those who were defeated in September not to stand, if only to avoid the label of bad losers. There are plenty of others – Chuka Umunna, Dan Jarvis, Rachel Reeves and Hilary Benn, to name the most obvious – who did not stand last autumn.

The chief problem is getting the PLP to act. Those MPs who have no chance of preferment, in cabinet or the shadow cabinet (and that is well over half of them) may believe that they will be safe in their seats, which matters more to some of them than winning the country. That belief is an illusion. Ask Nick Clegg, who is an authority on political disintegration.

The temptation will be to put off doing anything, in the hope “something” will turn up. The Micawber syndrome. But what is that something? Not Hilary Benn. Corbyn blinked, just as Harold Wilson did when he wanted to sack Benn’s father: the risks were too scary. Michael Dugher? Maria Eagle? Not enough for the gutless. Corbyn now knows he can’t survive without the PLP, but the PLP knows it can prosper without him. Delay is still the enemy. What further evidence of impending disaster do they need?

Others may feel that they will be accused of disloyalty to a leader whose parliamentary record is one of constant disloyalty. Of course, they will be accused of treachery by others who have made it a lifetime’s occupation. They should stiffen their spine and be guided by the words of the Elizabethan courtier Sir John Harington: “Treason doth never prosper: what’s the reason?/Why, if it prosper, none dare call it treason.”

Joe Haines served as press secretary to Harold Wilson from 1969-76. This article appears in the latest New Statesman magazine, The God Issue.