Opposition parties matter only inasmuch as they stake a claim to office. The weather is made by those who wield power or those with a reasonable expectation of winning it. Opposition is otherwise synonymous with irrelevance.

Jeremy Corbyn overturns the received wisdom. The record of Britain’s Labour leader is worse than dismal. No one imagines him as prime minister - not even, probably, Mr Corbyn. Yet he is already among Britain’s - even Europe’s - most consequential politicians.

Labour has just lost a by-election in the Copeland seat in the far north of England. The party had held it for 80 years. The win for Theresa May’s Conservatives marked the first time since the 1980s that a ruling party had stolen such a seat from the opposition. No one doubted Mr Corbyn was to blame.

The result was part of a pattern. Mr Corbyn’s personal ratings have been the worst of any Labour leader since the pollsters started counting. The party trails the Tories by up to 18 points in the opinion polls. Were Mrs May to call a snap election the projections suggest that her working majority of 17 seats would climb to more than 100.

Labour was always going to have problems during this parliament, not least because of the dreadful legacy of its last leader Ed Miliband who led it to needless defeat at the 2015 election. Mr Corbyn has since transformed a difficult position into a desperate one.

Chosen (twice) as leader by the party rank and file, he can claim the firm support of perhaps 10 or 15 of 220-odd Labour MPs. This does not seem to bother him. Mr Corbyn’s career has been lived on the far left fringes of politics, where ideological orthodoxy comes well ahead of a serious desire to wield power. He has never forgiven Tony Blair for winning three elections.

The way to understand Mr Corbyn is to place him in his time warp. The year is 1974. The war in Vietnam is drawing to a bloody close. With US help General Augusto Pinochet’s brutal regime has replaced Salvador Allende’s government in Chile. The CIA is plotting the violent overthrow of leftish parties everywhere. Richard Nixon is fighting to stay in the White House.

Here was a ready-made identity. To be progressive was to be anti-American - defenders of the oppressed against Washington’s imperialism, whether in Latin America, Indochina or southern Africa. Doubtless Mr Corbyn marched in protest to the American embassy in London. He has never stopped. Nothing better defines his politics than anti-Americanism. Hence the sympathy towards Vladimir Putin’s Russia, the reluctance to condemn Islamist terrorism, and the blind insistence that Hugo Chávez’s Venezuela was a success story.

International revolutionaries such as Mr Corbyn are not interested in the banal politics required to gather votes at home. Who can worry about housing, schools or transport, let alone the mundane aspirations of Middle England, ahead of the great liberation struggles? Who cares about Hartlepool when they are so closely acquainted with Havana?

At any other moment none of this would matter a jot. Labour would spend two, three or maybe four terms in opposition before finding itself an electable leader and moving back towards the centre. This is what happened during the 1980s and early 1990s. Four consecutive elections were lost before Mr Blair restored the party to power.

These, however, are not normal times. Britain is about to leave the EU - the most important, and dangerous decision it has taken since 1945. The government seems set on the hardest of hard Brexits. Why? Because Labour’s absence has turned politics into a conversation between Conservatives - a conversation in which English nationalists have the loudest voice.

The margin of victory in the referendum was sufficient to be indisputable. Yet had just 2 per cent changed their minds the vote would have been to remain in the EU. So it mattered that the leader of the opposition was a willing accomplice of the “outs”.

True, Mr Corbyn said he backed Remain. And then did nothing to prove the point. Labour’s strong pro-European voice, potentially decisive in the campaign, was muffled. The social democratic case for Europe - as a shield against the excesses of globalisation - went unmade. The referendum instead became a contest between a Tory prime minister David Cameron and the hardline Eurosceptics in his own party.

Much the same dynamic has played out in shaping the government’s negotiating strategy. Mrs May was on the Remain side, albeit never enthusiastically. But she has been driven since by the ideologues. Her stance now prioritises immigration control and curbing the European Court of Justice above economic prosperity. Business will be pushed out of both the single market and customs union and living standards will fall as a consequence. George Osborne, the former chancellor, says this may foreshadow the biggest protectionist shift in Britain’s history.

Some say Mr Corbyn’s weakness has left Mrs May one of the most powerful prime ministers of modern times. In matters European, the reverse is true. The absence of a credible opposition has left the prime minister a prisoner of those on her own side set on severing all ties with their own continent. There lies the paradox: Mr Corbyn is at once the most irrelevant and the most consequential politician of our times.

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