The Green Party has opened the door to an electoral pact with Labour if Jeremy Corbyn wins the party’s leadership race.

In an extraordinary open letter to Mr Corbyn, published in The Independent, the Greens' only MP, Caroline Lucas, fulsomely praised his campaign and suggested that the two parties should join together and not field candidates against each other in 2020.

But her endorsement will be controversial, not only in Labour circles but also her own party. The Greens saw their share of the vote increase by nearly three per cent at the last election – mainly at the expense of Labour and the Liberal Democrats.

Many Greens will be uncomfortable at the idea of standing aside to help Labour candidates win – especially given Mr Corbyn’s support for Britain’s coal industry.