The Green Party is on course to win a seat at Westminster, according to party leader Caroline Lucas.
In her keynote speech to the party conference in Brighton, she blamed Labour for the financial and political crisis facing the country.
"The government's response to the current economic crisis is creating more inequality, not reducing it," she said.
Lucas told the event that more than a million people voted for the Green Party in the European elections in June.
"A million people who responded to our policies, our candidates and perhaps, above all, to our values," she said.
"A million voices calling out for honesty in politics, for tackling the issues that really matter, like climate change and good quality public services.
"A million people who wanted fairness from a party they could trust. What a message for the other parties."
She added: "When the image of politics and political parties could hardly be any lower, we recently gained more than a thousand new members in just six weeks."
Lucas is the party's candidate for Brighton Pavilion at the next election, the seat seen as the best hope for a Green MP to be elected.
The constituency is a three-way marginal with the Conservatives also hoping to take the seat from Labour.
The Green Party now has as many Brighton and Hove councillors as Labour, and the party outpolled all the other parties in this year's European elections across all three of the city's parliamentary constituencies.
Lucas told the conference: "Here in Brighton Pavilion, people genuinely see that we're on course to win one of our first seats in Westminster at the next general election."
While she spoke at length about the financial crisis, reform of political institutions and the economy, environmental policies were at the heart of the speech.
"We demand an end to airport expansion, and to plans for a fleet of new coal fired power stations," she said.
"We demand a massive investment in energy efficiency, renewable energy, in public transport and green jobs.
"And we demand that when the government signs up to new emission targets, they undertake to make domestic cuts here at home, not to outsource or offset them in poorer countries.
"Grassroots mobilisations like Climate Camp and the 10:10 campaign, launched this week, are vitally important.
“But commitment and engagement amongst civil society must be matched with real political leadership.
"In a few years, people will look back bewildered and angry that - knowing what they knew now - none of the other main political parties in Britain confronted the most critical issue of our time.
"They have pretended that they have the problem under control. That a few low energy light-bulbs here, a bit of lagging on your loft there, and the problem is solved.
"And that to do anything more is either unnecessary or involves too much 'sacrifice'.
"We've got news for them: a transition to a post-carbon world doesn't have to be about sacrifice.
"It's about jobs, it's about a more equal society, and it's about a way of life with the potential to be far more fulfilling than the turbo-charged consumerism which is peddled by politicians today.
"And that's why we say that our government's inaction is nothing less than a political crime."
