Labour is a party torn between its parliamentary and activist wings. Since Jeremy Corbyn, who this week appealed desperately for unity, was re-elected by a landslide last September, Labour has become the first opposition in 35 years to lose a ­by-election to the governing party and has continually trailed the Conservatives by a double-digit margin. Yet polling suggests that, were Mr Corbyn’s leadership challenged again, he would win by a comfortable margin. Meanwhile, many of the party’s most gifted and experienced MPs refuse to serve on the front bench. In 2015 Mr Corbyn made the leadership ballot only with the aid of political opponents such as Margaret Beckett and Frank Field. Of the 36 MPs who nominated him, just 15 went on to vote for him.

Having hugely underestimated the strength of the Labour left once, the Parliamentary Labour Party (PLP) will not do so again. In the contest that will follow Mr Corbyn’s eventual departure, the centrists could lock out potential successors such as the shadow business secretary, Rebecca Long-Bailey. Under Labour’s current rules, candidates require support from at least 15 per cent of the party’s MPs and MEPs.

This conundrum explains the attempt by Mr Corbyn’s supporters to reduce the threshold to 5 per cent. The “McDonnell amendment” (named after the shadow chancellor, who failed to make the ballot in 2007 and 2010) is being championed by the Bennite Campaign for Labour Party Democracy and Jon Lansman of Momentum, who is interviewed by Tanya Gold on page 34. “For 20 years the left was denied a voice,” he tweeted to the party’s deputy leader, Tom Watson, on 19 March. “We will deny a voice to no one. We face big challenges, and we need our mass membership to win again.”

The passage of the amendment at this year’s Labour conference would aid Mr Lansman’s decades-long quest to bring the party under the full control of activists. MPs have already lost the third of the vote they held under the electoral college system. They face losing what little influence they retain.

No Labour leader has received less support from his MPs than Mr Corbyn. However, the amendment would enable the election of an even more unpopular figure. For this reason, it should be resolutely opposed. One should respect the motivation of the members and activists, yet Labour must remain a party capable of appealing to a majority of people, a party that is capable of winning elections.

Since it was founded, Labour has been an explicitly parliamentary party. As Clause One of its constitution states: “[The party’s] purpose is to organise and maintain in Parliament and in the country a political Labour Party.” The absurdity of a leader opposed by as much as 95 per cent of his own MPs is incompatible with this mission. Those who do not enjoy the backing of their parliamentary colleagues will struggle to persuade the voters that they deserve their support.

Labour’s divisions have rendered it unfit for government at a moment of profound political change. Rather than formalising this split, the party needs to overcome it – or prepare for one of the greatest defeats in its history.