Labour conference has overwhelmingly backed a radical bid to work towards slashing carbon emissions by 2030 amid pressure from activists to toughen the party’s stance.

Delegates voted for a motion by campaigners for a Green New Deal which “works towards a path to net zero carbon emissions by 2030”, guarantees green jobs and calls for nationalisation of the big six energy firms.

The GMB union, which represents energy workers, strongly opposed the accelerated target, instead supporting a separate motion to cut carbon emissions without a fixed date.

Both motions were passed by delegates but support for the more radical bid will heap pressure on the party leadership, as conference motions are used to help shape Labour policy.

A 2030 target would be one of the most ambitious in Europe and brings Labour in line with the Green Party, but ahead of the Tories, who are aiming for 2050, and the Liberal Democrat target of 2045.

Labour’s current commitment is to meet net zero before 2050, but John McDonnell revealed in the summer that serious thought was going into reducing the deadline by two decades.

Theresa May announced plans to introduce a legally binding target to end greenhouse gas emissions by 2050, following recommendations from the government’s independent climate change advisers.

The move comes after influential trade unions rowed in behind the more ambitious decarbonisation target, including Unison, Unite and CWU.

She told a fringe event at Labour conference: “I think getting there as quickly as we can, by 2030, provided – and I made this point to the room and this is reflected in the motion that we got which is great – provided that we’ve got a credible plan to back it up and provided that social justice is at its very heart to protect workers and ensure there’s a just transition.”

Neil Derrick, from GMB, was booed by delegates when he argued there was “no credible plan for achieving zero carbon by that date”.

He added: “The implications would be swingeing and it would affect everyone.

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“It would require the confiscation of all petrol-fuelled cars still on the road, the state-rationing of meat, limiting families to one foreign flight every five years, the closure of whole industries.”

Delegates spent hours hammering out both motions ahead of the votes on Tuesday, which insiders say were the longest sessions in Labour conference history.

Lauren Townsend, trade unionist and spokesperson for Labour for a Green New Deal, said: “The Labour movement has voted to take leadership on the climate emergency with a response which puts people and planet before profit.

“Now the ambition has been set, it is time for our movement to come together to build a Green New Deal from the ground up in every town, village and city.”

John Sauven, executive director of Greenpeace UK, said: “Net zero by 2030 will be extremely difficult, but it may the right date to aim for.

“If it can be done, it should be, and if it can’t, then missing the target by a few years, or even a decade, is still a far better outcome than hitting the government’s 2050 target, which is dangerously late.”

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Labour insiders made it clear that the party supports both motions, which it does not regard as contradictory.

A party spokesperson said: “Labour now has the most ambitious net zero plans in the world.

“Conference agreed that these should be achieved through massive investment in infrastructure and skills, public ownership of key utilities and supporting climate transition in the global south.

“Labour’s plans for a Green New Deal will bring about a green industrial revolution which is bold, credible and has climate justice at its heart.”

Business secretary Andrea Leadsom seized on concerns by unions, saying even Labour supporters were admitting the plan is “completely unworkable and would threaten jobs”.

She said: “Labour’s plans to renationalise huge swathes of the energy system would scare away the investment that is so vital for reducing emissions, while leaving the taxpayer to pick up the huge cost.