The Green Party is surging in the polls and co-leader Sian Berry says candidates are poised to win seats and take on Nigel Farage's Brexit party in a general election.

Berry said the party would not automatically seek an electoral pact with bigger left-wing parties as it did in 2017.

She said the Lib Dem leadership frontrunner Jo Swinson's defence of austerity illustrated why the party was wary of any "progressive alliance" with other parties.

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LONDON — The Greens are having a good year.

In May's local elections, the party more than doubled its council seats to 362, a bigger proportional gain than any other party. In the European elections that followed, the party more than doubled their seat count, from three to seven, on a fiercely anti-Brexit platform.

Sian Berry, who co-leads the party with Lambeth Councillor Jonathan Bartley, said the party's recent successes are down to an organised ground campaign, their opposition to austerity and climate change, and their vocal opposition to Brexit.

"People were looking to vote for parties who supported remain or leave depending on their preference to send a signal and we had a very, very strong remain message," Berry said.

"We're very coherent and we're very clear in our messaging, and that really helps people to notice us. And it's a proportional election, people know when they vote for us it counts."

The Greens have normally had a harder time in general elections, where the UK's "first past the post" system typically favours more established parties and deters some voters from supporting marginal ones.

However, Berry says the Greens' recent successes prove the party is no longer one of just protest votes.

She believes they are ready to make significant gains at the next general election, with national polls putting the party's support on up to 9%, and indeed says their candidates could be the ones best placed to fight off Nigel Farage's Brexit party in constituencies where voters' views on Brexit are polarised.

"We're coming into an election with more strength than we've ever had before, with real prospects of winning more seats under first past the post," Berry said.

"We were first [in European elections] in Bristol, Norwich and Brighton which are all areas where we've got a good record in parliamentary elections, and we were second behind the Brexit Party in Leave-voting places like Sheffield, Lancaster and Hastings."

Berry cast doubt on the prospect of another "progressive alliance" with other left-leaning parties in the event of a snap election — something the party sought in 2017.

She highlighted the Liberal Democrats' record on austerity and said there was a risk of other parties using her party's support to "greenwash" their own policies.

"We can't be helping to greenwash parties who just want to give lip service to climate change, and we can't be helping parties who still want to press on with austerity," Berry said.

She highlighted Liberal Democrat Jo Swinson's defence of the party's record on austerity, who is poised to replace Vince Cable as Liberal Democrat leader this summer.

"Jo Swinson hasn't disavowed austerity, she hasn't apologised for it. She said it was necessary. We do need to make sure we're not being used to greenwash anything. So those are things to talk about in any wider progressive pact."