The Green Party is seeking an electoral pact with Labour in the event of Jeremy Corbyn becoming the party's leader.
In an open letter published in The Independent, Caroline Lucas, the only Green MP, praised the Corbyn campaign and suggested the parties do not field candidates against each other in 2020.
The old politics is crumbling, not just in Britain but across our continent. We now have the chance to embrace a movement based not on greed or fear, but on resilient local communities, people working together and a stable economy that works for generations to come. I truly hope you win the contest on 12 September – and I look forward to continuing to work with you to bring about the progressive politics that has inspired us both for so many years.
The idea of a Green-Labour alliance is hugely controversial. The Greens increased their vote share at the last election by three per cent largely at the expense of Labour and the Liberal Democrats. Mr Corbyn is also an avowed supporter of Britain's coal industry.
You can read the full letter here.