The Green Party will not collaborate with the Liberal Democrats in any general election pacts, because Nick Clegg and his ministers have become so discredited on the environment, its leader said yesterday.

Last week, the Greens overtook the Lib Dems in a poll which put them fourth behind Labour, the Conservatives and Ukip, raising the prospect that Mr Clegg's party could be forced to make informal non-aggression pacts to prevent an electoral wipeout.

In an interview with The Independent on Sunday, Natalie Bennett said the 2015 election has been thrown wide open and Britain is now in a state of six-party politics – given the surge for the SNP at the expense of Labour in a separate poll last week. But she said a pact with the Lib Dems was "not going to happen" because they had "blown their green credentials" by backing nuclear power and fracking with their coalition partners.

The Green Party is fielding candidates in at least 75 per cent of seats next May. Its top 10 target list includes Lib Dem and Labour constituencies and Ms Bennett said she was optimistic that there's a new generation of voters who were considering supporting her party.

"The trend [in the polls] is the most pleasing thing and is tapping into a rise in membership and support," she said. "People have had a little bit more of a chance to hear from us. Because of the issue of the leaders' debates we have had a bit more coverage.

"People are really engaged and liking what they are hearing. Once people have had more chance to hear what we stand for and hear the message, we hope to get more support.

"With this election … British politics will no longer be fought along old battlelines but on a seat by seat basis. People are giving up their old loyalties. I think people aren't following them in the way they have in the past. They are looking at the seats, looking at the candidates and making a judgement based on that. The old two-horse race is over." Ms Bennett said the Greens would be targeting seats where they had strong local parties, there were good local issues and arguments and "where people are looking for change".

"The notion of politics being local – that is something the Green Party has been very much at the forefront of. It is really important to speak to people about their locality, particularly when thinking about economy," she said.

Growing support for the Greens meant it was "disappointing" that she has not been invited to take part in the BBC general election leaders' TV debate, Ms Bennett said. "The nature of their arguments is that the BBC is starting from 2010. But we are in a profoundly different place to 2010. In 2010 the financial crash had just happened. Now we have got an economic crisis, an environmental crisis and a social justice crisis. And the BBC is contributing to the political crisis by their response.

"The change since 2010 is more fundamental than [the rise of] Ukip, it is grounded in the environment and the economy. On the economy, people are hearing the Government saying we have had a recovery but people are not feeling a personal recovery for themselves. There is disillusionment with the neoliberal way of thinking on the environment. People are looking for new ways of thinking for the world."

Last week the Lib Dem MP Andrew George, who has a majority of 1,719 over the Conservatives in St Ives, suggested he would support an electoral pact between his party and the Greens. At the 2010 election, the Green vote was 1,308 votes but a surge in support next May could mean he loses his majority.

Ms Bennett added: "In local areas if there is a strong local residents' group or the NHA [National Health Action] Party, then we would consider it, but of the three or four main parties I cannot see any way that that would happen."

She said the Lib Dems' backing for shale gas and new nuclear power stations meant "they have just blown their green credentials … we and the Lib Dems have fundamentally different approaches to the world and different views.

"The Lib Dems are fundamentally a neoliberal party, comfortable with society being run for the benefit of the richest 1 per cent of society.

"David Cameron is a man who appointed Owen Paterson as Environment Secretary – he must have known about Mr Paterson's views on climate change."

Tim Farron, the Lib Dem president, has also said there should be no pact with Green Party candidates.

The Green vote has risen by 3 per cent in the polls in the past six months. A YouGov poll last week put them on 7 per cent, up two points and one point ahead of the Lib Dems.