Until a few months ago, Labour thought that its passage to power was simple enough. Hold onto its core vote, scoop up angry left-wing voters disaffected with the Liberal Democrats, and watch Ukip and the electoral system do the rest. They were reckoning without the Greens.

In 2010, the Green Party of England and Wales and the Scottish Green Party amassed a combined 1 per cent of the vote. Today the Greens are averaging 5 per cent, and even reached an all-time high of 8 per cent in a YouGov poll last week. And their trajectory is only going up: their average support has doubled since April. They are rapidly gaining footsoldiers – the membership of the Green Party of England and Wales has risen by 90 per cent in 2014. For so long bereft of publicity, the Greens have astutely exploited their exclusion from the broadcasters’ proposals from TV debates.

The upshot is terrifying for Labour. A Green surge would shatter Labour’s fragile electoral coalition for good. No wonder David Cameron supports including the Greens in the TV debates.

Labour is alert to this threat. Last month it appointed Sadiq Khan to lead a Labour’s Green Party Strategy Unit. Khan seems to believe that the Greens can be flattened through flattery. This week, he praised Caroline Lucas “with whom I agree on a great many things” and Green supporters, who “share the same values and aims as the Labour Party: reducing inequality, saving the NHS, building more homes, a commitment to human rights and civil liberties and protecting our environment.” But he warned that “every vote for the Green Party only makes it one vote easier for the Conservatives to win.” Vote Green, Get Tory is the new Vote Ukip, Get Labour.

The great problem for Labour is that it cannot simultaneously launch an offensive on its left and right flanks. Before 2013, Ukip took only one Labour vote for every nine they took from the Conservatives: no wonder Labour seemed so unperturbed by their rise. Since January 2013, it has lost six voters to Ukip for every nine that the Conservatives have lost to the People’s Army. Labour can try – as it has, but to no avail – to win back Ukip defectors but reconciling this with reaching out to disaffected left-wingers flirting with the Greens looks like an impossible balancing act.

And the Greens want to make it harder still. At last night’s Leaders Live debate, when Natalie Bennett answered questions from young voters, the Green Party leader again positioned her party well to Labour’s left. She reiterated her support for a wealth tax, and said she was attracted by a top rate of income tax of above 50 per cent, which would be imposed on income earned over £100,000, rather than over £150,000 as Labour proposes. Bennett also reiterated her support for the abolition of all academies and free schools.

Throughout, the implication was clear. Where Khan called Labour “a truly radical party again”, Bennett was decrying them as vacillating supporters of timid and incremental change. Labour’s worst nightmare is that enough of the Lib Dem defectors it has been relying on agree.

The Greens remain a long way from being a true “Ukip of the left”. They are yet to develop much working-class appeal – 61 per cent of its supporters are ABC1. Unless that changes drastically, they will not match Ukip in the election. But that does not mean they could not have a critical impact on the next general election: even with just three per cent of the vote, Ukip cost the Tories at least five seats in 2010. So perilous is Labour’s route to Downing Street that even similar damage to them from the Greens next May could stop Ed Miliband becoming Prime Minister.