The resurgent Green party is to target a dozen seats across England, which it believes it could either win or come close to seizing in next May’s general election, as membership rises and confidence grows that it could outpoll the Lib Dems.

Last week, the number of Green party members in England and Wales passed 22,000 – a 57% increase since 1 January – with the number of young Greens (under-30s) having risen 100% to more than 8,000 since 1 March.

Party leader Natalie Bennett told the Observer that the weakness of the Tories, Labour and the Liberal Democrats, and the rise of Ukip, all pointed to the next election being the most difficult to predict of recent times, and one in which the Greens could realistically step in and gain half a dozen seats or more.

“I think there is a chance that, in 2015, everything just breaks wide open. With the rise of Ukip, not much more than 25% of the vote could win a seat,” she said.

The Greens, who currently hold only one seat, Brighton Pavilion, have been steadily creeping up in the polls over recent months. Most national surveys have put them between 4% and 7%, within touching distance of the Lib Dems, who have been struggling to get back into double figures, despite having won 23% of the vote at the 2010 election.

Last week, the party said it would take legal action unless it was included in TV debates, arguing that it received 150,000 more votes than the Lib Dems in the 2014 European elections and won three times as many seats as them.

Bennett said that as well as defending Brighton Pavilion, where ex-leader Caroline Lucas is the incumbent, the national party has its eye on another 11 seats in England, and it believes winning around six is not out of the question. “I refuse to predict, but I think there is potential for that range [six seats],” she said.

Bennett put the recent increase in the Greens’ membership down to disillusionment with the three more established parties and their leaders, and to a sense among more and more voters that they want to back a party they believe in, even if it remains small. “People see Cameron, Miliband and Clegg and say, ‘Well, that is not it.’ They are looking around for something new.”

Most Green converts have come from Labour and the Lib Dems, she says. “With the Lib Dems, it is [because of] tuition fees, nuclear power, nuclear weapons and civil rights issues like secret courts. With Labour, it is different. It is that it is still a neo-Thatcherite party and it is still New Labour with the thinnest of rhetorical glosses over the top.” But there also some disappointed Green Tories who, she argues, don’t like fracking and building on the green belt.

As the Greens have gained more media attention, Bennett has thought seriously about post-election possibilities, and what role her party might play in supporting a Tory- or Labour-led government. “I can’t imagine circumstances in which we would prop up a Tory government,” she says. “Our first inclination would be a ‘confidence and supply’ agreement, rather than a coalition, because it means you provide stable government – you don’t get the ministerial cars but you keep your conscience and you don’t have to vote for tuition fees, for example.”

She is marginally warmer about an alliance with Labour, again probably not in a formal coalition. “We are not enthusiastic about Labour but the choice between [Labour and Tory] is obvious.”

The Greens, she says, will be pushing several policy priorities, including plans to lift the minimum wage to £10 by 2020, the renationalisation of the railways when franchises come up for renewal, greater funding for improved home insulation, and the reversal of creeping privatisation of the NHS.

Top of the target list will be Norwich South, currently held by the Lib Dems with a wafer-thin majority over Labour, but where the Greens took 14.9% in 2010. A recent poll by Lord Ashcroft put the Greens on 20% in the constituency.

Another possibility is Bristol West, where the Lib Dem incumbent, Stephen Williams, is likely to struggle because of the fall-off in the student vote. Then there is St Ives. “Cornwall is very interesting because there is basically no Labour party there, only Tory and Lib Dem, yet it is a place with incredibly low wages and seasonal casual employment so, in the county elections, we got an average of 18% of the vote,” says Bennett.

Others on the target list are Sheffield Central (Labour), Liverpool Riverside (Labour), Oxford East (Labour), Solihull (Lib Dem), Reading East (Tory), York Central (Labour), Holborn and St Pancras (Labour) – where Bennett is standing as the candidate – and Cambridge (Lib Dem).