The Government was facing a growing credibility gap over green jobs last night as environmental campaigners and trade unionists united to fight the closure of Britain's sole major wind turbine plant.

Only last week, ministers proclaimed a green employment future for the UK involving 400,000 jobs in environmental industries such as renewable energy – yet this week they are declining to intervene over the forthcoming closure of the Vestas Wind Systems plant on the Isle of Wight, with nearly 600 redundancies.

Workers at the Newport factory, which makes wind turbine blades, were last night staging their third night of occupation of the plant in an attempt to prevent the closure which is scheduled for 31 July. In an alliance not seen before, they were being helped by climate-change campaigners who have set up an ad hoc camp outside the factory and yesterday helped to get food to the occupiers.

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Vestas, a Danish company which is the world's biggest wind energy group, announced in April it was pulling out of the UK, citing the difficulties of getting wind farms built in Britain in the face of local "Nimby" opposition campaigns and the slowness of the planning system.

"A problem we are facing is our inability to get planning consent," said a senior company executive. "We needed a stable long-term market and that was not there in the UK. We have made clear to the Government that we need a market. We do not need money."

Several weeks before the closure announcement, Vestas bosses led by the chief executive, Ditlev Engle, went to 10 Downing Street for a high-level meeting attended by the Energy and Climate Change Secretary, Ed Miliband, where they made specific demands for more direct government support. When this was not forthcoming, the closure was announced.

While the Government may not have felt able to respond to what were in effect threats from a private manufacturing company, the consequence of allowing the country's major wind energy manufacturing plant to fold has attracted ferocious criticism from the green movement.

This was not least because of the prospect that the 7,000 or so wind turbines Britain will install over the next decade to help meet its climate-change targets will have to come from abroad, even though last week both Mr Miliband and the Business Secretary, Lord Mandelson, launching the Government's climate strategy, went to great lengths to stress the green business opportunities of Britain becoming a low-carbon economy.

"Last week Labour promised Britain would install thousands of wind turbines in the coming years. Are ministers really now saying they'd rather buy those turbines from abroad than make them here in the UK?" said Robin Oakley, head of the Greepeace climate campaign. "Letting this factory close is like a football manager saying he's up for the cup then dropping his only goal scorer. It just doesn't make sense.

"It is factories like this and engineers like the ones occupying it that Britain desperately needs if ministers are serious about launching a green industrial revolution."

Caroline Lucas, the Green Party leader and the Isle of Wight's MEP, sent a message of support to the workers and called for immediate government intervention to save the factory from closure. "The decision to close the facility represents a spectacular failure by government ministers to adequately promote green industries, and protect the future of manufacturing in this country," she said.

Nick Clegg, the Liberal Democrat leader, said: "This closure exposes the hollow truth of Labour's climate change strategy." Five Labour MPs have already signed a Commons motion protesting against the closure.

A spokeswoman for the Department of Energy and Climate Change said the company had taken a commercial decision to reduce its production capacity across northern Europe. She acknowledged there were "cultural and planning issues" behind the construction of wind farms, but promised they would be tackled by the climate strategy. She said: "We are hopeful Vestas will go ahead with their plans for a research and development facility on the Isle of Wight which could provide up to a further 300 jobs and also help develop and test products that are suitable for the UK offshore market."

Vestas erected a fence around the site in response to the protest. Workers claimed it was being put up to stop food or drink being sent in. One said: "We are convinced this is against the Human Rights Act because we are being denied humanitarian aid."

Three protesters were arrested outside the site. Hampshire Police said a 28-year-old man from Southampton had been arrested on suspicion of assaulting a police officer and a 49-year-old man from Portsmouth was arrested on suspicion of a breach of the peace.

A London man, aged 38, was arrested on suspicion of a breach of the peace and later released without charge.