There was an important political milestone last week — and I don’t mean the election of Ukip’s first (defecting Tory) MP — Green Party membership reached an all-time high of 26,000 in the UK.

I am proud that the party I joined way back in 1980 when I was just 14 has broken all previous membership records.

The Scottish Green Party became independent in 1990 and last time I looked was at 6,000 members.

As a supporter of the Yes campaign, it doubled its membership since the independence referendum.

Meanwhile the Green Party of England and Wales broke the 20,000 barrier at the start of October.

Despite being a lifelong member of the party, I am aware that my party like any other is far from perfect.

All political parties on the left have both strengths and weakness.

Politics isn’t football — supporting a team is sport but politics is about the most effective way of promoting social change, and no-one organisation has a monopoly in this regard.

Nonetheless I am proud of the Greens. I have never seen our party stronger and equally I have never seen our party stronger in promoting equality and justice.

From the Economist to the Guardian and the left press, rising Green Party membership has been linked to a move to the left under our MP Caroline Lucas and our recently re-elected leader Natalie Bennett.

The Green Party is increasingly identifying as a party of the left.

Bennett and Lucas are often on the picket lines supporting low-paid workers.

Lucas is a president of the PCS union and the Greens are campaigning strongly for a set of left policies including a wealth tax, renationalisation of rail, rent control, a living wage and stronger trade union rights.

The Green Party was one of the first organisations to draw attention to TTIP, a trade and investment treaty that would allow corporations to sue governments.

TTIP would rewrite the rules to make it impossible to protect workers and the environment, and the Green Party of England and Wales has been organising demonstrations and protests against since spring this year.

Unlike the Tories, Lib Dems and Labour, the Greens have opposed British intervention in the Middle East but strongly support the Kurds in Rojava who have been fighting the so-called Islamic State.

A green politics that fails to champion social justice or to see the drive to corporate profit as a key source of environmental destruction, which the Green Party recognises, makes little sense.

The party has increased in membership by nearly 50 per cent during 2014 — remarkable growth by any standard.

Surveys of new members show that they are overwhelmingly on the left.

A check on way people have joined the party throws up comments like “I joined because the Green Party is firmly on the left,” “The Green Party has policies for social justice,” and “I joined the Greens because I am a lifelong socialist.”

There have, of course, always been socialists in the Green Party.

I am a veteran of such eco-socialist groups as the Association of Socialist Greens, Green Revolution, The Way Ahead and Green Left.

There is a strong eco-socialist tradition in Britain.

The first British socialist political party, the Social Democrat Federation, was founded in 1884 by the designer and poet William Morris.

Morris became a socialist because of his concern with conservation and horror at the pollution of smoggy Victorian London.

The Welsh literary critic, novelist and former World War II tank commander Raymond Williams used the term “eco-socialist” and wrote an essay on red-green politics entitled Socialism and Ecology in 1982.

It was published by the Socialist Environmental and Resources Association (Sera).

Founded in the 1970s, Sera, which is now affiliated to the Labour Party, contained eco-socialist from a wide variety of organisations.

Eco-socialists writers and organisations have done a great job in promoting green-left politics but the increasing left nature of the Green Party of England and Wales is the product of wider forces and factors.

The student protests against fees saw the Young Greens move to the left.

Leading party members like Lucas have increasingly seen that social issues are essential to green politics.

 Some reasons are less positive. The unrelenting attacks of Britain’s tabloids along with Thatcher’s class war on behalf of the super-rich have shifted British politics in an increasingly ugly direction.
 
Labour has shifted to the right, and the far left have diminished, so there is political space for a left alternatives.

Increasingly voters are attracted to the Greens not only because of the strength of green policies but by the lack of viable alternatives to neoliberal politics as usual.

All three main political parties have increasingly failed to defend civil liberties or support the benefits of an outward looking multicultural society, so Greens are picking up votes among those who reject a reactionary politics.

Many commentators are calling the Greens the Ukip of the left.

I don’t think this fits our political vision, which is about critical thinking about difficult problems such as climate change and social justice, but it does indicate that we are increasingly posing an alternative and challenging the main three parties.

Sustaining our growth in membership and deepening our policies on social justice will create a number of challenges.

New members need to feel part of our political community.

Local Green parties need to be both effective electoral machines and to campaign on issues such as TTIP, and be able to involve new participants.

Green politics cannot be reduced to a few key policies but is a sophisticated body of political thought.

Political education is necessary but must be based on dialogue with new members not top-down edicts.

Equally British politics is tough.

The British elite have hundreds of years of practice at disarming radicals and keeping power concentrated in their own hands, so any radical political party that grows will face pressure.

Our electoral system is far from fair to electoral challengers.

However we Greens have managed to elect a brilliant MP, three MEPS and over 150 councillors.

Above all, those of us who have been members for decades are going to need to adapt.

The Green Party used to be the Ecology Party.

But our party cannot afford to have a green wing and a socialist wing — ecological politics needs social change and social justice is impossible with a degraded planet.

Many of the best philosophers have remind us that far from being separate from it, human beings are part of nature.

To protect our environment we need social justice.

Green has to be left but it must not forget the importance of ecological challenges.

Climate change is accelerating, and a recent survey from the World Wildlife Fund says 40 per cent of species are under threat.

I am glad the Green Party is growing but our recent success is just a small step in the direction we need.

Derek Wall is international co-ordinator of the Green Party.