The Green party is positioning itself as the “real opposition” to the coalition, opening up a bitter fight with Labour on the left wing of UK politics, as the party’s key figures prepare to contest general election seats in parliament.

Caroline Lucas, the party’s only MP, will tell the Green party conference in Birmingham on Saturday that Labour has failed to oppose the Tories and Liberal Democrats on a wide variety of issues, from the creeping privatisation of the National Health Service to free schools, tougher immigration rules and the renationalisation of the UK’s rail network.

“Labour has not only consistently failed to challenge the illiterate economics of George Osborne’s slash and burn approach to public spending, but Ed Balls [shadow chancellor of the exchequer] has signed up to the same spending limits as the government itself,” she will tell party activists gathering at Aston University as the party conference season kicks off.

“Secret courts as part of the justice and security bill? Labour refused to oppose [them]. On the appallingly illiberal immigration bill, they abstained. They support workfare sanctions. Even on the issue of bringing the railways back into public ownership – a hugely popular policy – Labour has flunked it.”

Environmental issues are to be barely given a mention in her keynote speech, which will concentrate on social justice, inequality and civil liberties. She will slam the Tories for “talking openly of getting rid of green crap, offering tax breaks for fracking, boasting of getting every last drop of oil from the North Sea, even as climate change accelerates”, and reiterate the Green party’s opposition to fracking for oil and gas and to nuclear power and weapons. But most of her speech is reserved for strident criticism of Labour’s current spell in opposition.

Lucas holds the Brighton Pavilion seat in Westminster, the party’s first, gained at the last general election after years when the party scored well in European elections, with more than a million Green votes cast, and some local elections, but had historically failed to make a dent in Westminster.

But her seat will be strongly contested by Labour in next May’s general election. Labour is understood to view the Brighton seat as one of its most winnable.

Another Labour stronghold, the inner London constituency of Holborn and St Pancras, is also a key Green target. The party’s leader, former Guardian journalist Natalie Bennett, will stand there in the hopes of an upset following the retirement of the long-standing Labour MP Frank Dobson.

Every seat will be crucial for Labour in next year’s election, as polls continue to show only a narrow gap between the two main parties, with a strong chance of another hung parliament. Support for the Lib Dems has retreated since they took up the junior position in the current ruling coalition. A vote for Scottish independence would throw the party’s strategy further into doubt, as it would raise the prospect of the loss of Scottish Labour seats, which would be a serious blow to hopes of forming a government.

Labour will also face a fight on the left from the Green party in seats in Manchester, Oxford, Bristol and Norwich.

Lucas will say she wants to “demonstrate that our result in Brighton in 2010 wasn’t a one-off”. She will also point to the recent successes of Ukip, in an effort to galvanise voters to support her party. “They are knocking on the door of parliament. Imagine if they are in the next parliament and we are not. What a terrible message that would send out about the direction in which this country is going.”

Bennett, who will speak on Friday afternoon, will tell the conference that voter apathy and low turn-outs reflect a lack of distinct policies among the main parties. “Voters are desperate for alternatives to the three business-as-usual parties. Voters understand our current model is broken, our economy and society are failing to meet our needs. The coalition has governed for those who think prestige and personal wealth is more important than fairness and a decent life for everyone. This government has allowed giant, tax-dodging, low-paying, exploitative multinational companies to act at the direct expense of individual workers and communities.”