Each of the three establishment parties has succeeded in alienating its core vote. Labour over Iraq and the casino banking culture that flourished during its tenure in office, the Tories over Europe and equal marriage, and the Lib Dems over tuition fees and propping up of one of the most anti-egalitarian governments of modern times. All have been tainted by the scandal over MPs' expenses. As a result, participation in mainstream politics is declining further than ever.

The UK's first-past-the-post voting system is said to produce strong governments, avoiding what many perceive as the grubby infighting that dominates politics on the continent. But it isn't working anymore. Millions of votes don't count in rock solid safe seats and supporters of small parties are unrepresented or under-represented in parliament.

Many voters damn the political elite with the familiar refrain: "They're all the same." This is fairly true with regard to the big three parties: Labour, Tory and Lib Dem. There is very little difference between them these days. They all embrace, to marginally varying degrees, neo-liberal economics.

Many people are, however, desperate for an alternative but they fear their voice will not be heard.

The European elections this Thursday offer a chance for something different. Because they use a system of proportional representation (PR), we have an opportunity to vote for what we believe in, without fearing that our votes will be wasted. PR is sometimes a mixed blessing. It was PR that allowed Ukip a foot in the door at the last Euro poll, and in this election it looks like the anti-EU party will win more seats than anyone thought possible for a new party 15 or even 10 years ago.

Nigel Farage entered the European Parliament in 1999. This was also the year that Caroline Lucas was elected as one of the UK's first two Green MEPs (the other was Jean Lambert). She went on to become the first Green MP at Westminster. A parliamentary seat still evades Farage and his party.

Ukip supporters want to withdraw from the EU. They fantasise about plucky Britain standing alone against the world. Ukip stirs this nostalgia for 'Great Britain' and excites fear about immigrants and refugees. It has filled ​some of the ​the void created by the discredited mainstream politics and, in particular, by the weakness of the orthodox left.

But for people who believe in social justice and equality, and who want action to thwart climate destruction and to protect the precious environment on which all life depends, the Greens - not Ukip - are the real alternative to the big three parties.

The Green vote is seen by some people as a protest vote, and there is no reason why it shouldn't be. It is a vote against Labour's failure to defend working class people and its initiation of the part privatisation of education and health care. It is a vote against the Lib Dem's abandonment of principle in favour of power. It is a vote against Tory austerity which makes ordinary people pay for the economic crisis created by reckless bankers. It is most certainly a vote against the homophobia, xenophobia and climate change denial of Ukip.

But in this election, voting Green it is also a vote for something. The Greens are a party that offers an imaginative, alternative positive vision of how our future could look. This is fairly unique, given the broad political consensus between the stale, grey Tories, Labour and Lib Dems.

Unlike the three Establishment parties and Ukip, the Greens advocate decisive EU action to close tax avoidance loopholes and tax havens, tax empty homes and financial transactions, cap banker's bonuses, axe nuclear weapons, prioritise energy conservation to cut household bills and to introduce rent controls, a living wage and free education.

As a veteran of nearly 50 years of political campaigns, I look toward 22 May with a strange mixture of hope and fear. Fear that the hate-mongers of Ukip are poised to advance and to challenge some of the gains in minority rights and human rights, with the aid of their far right allies in the European Parliament. But also hope that the Greens may eclipse the Lib Dems; including the election of new Green MEPs such as Peter Cranie in North West England and Rupert Read in the East of England. Both lost narrowly last time. A tiny swing to the Greens will get them elected and, in the North West, will have the added bonus of probably surpassing the British National Party vote and thereby blocking the re-election of BNP leader Nick Griffin.

Make sure you vote: Show Ukip and the three Establishment parties the red card. Give the Greens a chance.