The Greens would look to back a Labour minority government on a "case-by-case" basis, a senior party source told IBTimes UK on Wednesday 7 June. The news comes just a day before Britain goes to the polls in the general election on 8 June.
The Conservatives started off the campaign with double-digit leads in the opinion polls, but some companies, such as YouGov, have said the Tories are likely to be short of a majority in the House of Commons after the vote.
Surprise Labour gains could see Jeremy Corbyn attempt to form a government on a coalition, or so called supply and demand basis, where minority parties back the government on key pieces of legislation.
The Liberal Democrats, who are defending nine seats at the election, have rejected a coalition, while Corbyn said he would be "open" to talks with SNP leader Nicola Sturgeon over a second Scottish independence referendum.
The Greens, meanwhile, have called for a tactical-voting Progressive Alliance at the election. The move has been backed by former Labour shadow cabinet minister Clive Lewis, but Corbyn and Farron rejected the plan.
"We'd obviously never back the Tories and certainly look to support Labour on a case by case basis," a senior Green source told IBTimes UK.
The party notably disagrees with Labour over the renewal of the UK's nuclear deterrent, Trident, which it wants to scrap, and over immigration policy. The Greens back the free movement of people from the EU, but Labour has said free movement would end once the UK splits from the EU in two year's time.
The Greens are defending just one seat at the election, with co-leader Caroline Lucas contesting Brighton Pavilion. Green MEP Molly Scott Cato is also hoping to take Bristol West from Labour, who are defending a majority of more than 5,500 votes from 2015.
The latest opinion poll from from Opinium, of more than 3,000 people between 4 and 6 June, put the Conservatives on 43%, Labour on 36%, the Liberal Democrats on 8% and Ukip on 5%.