Ed Miliband has vowed to build a “high-ambition coalition” of UK businesses, trade unions and civic society to challenge the government’s “backward” environmental policies.

The former Labour leader said the agreement reached at the UN climate talks in Paris this month provided a historic opportunity to tackle climate change. But he said time was running out.

“This is the thing my kids will judge me on,” Miliband told the Guardian. “Did you do something about this? Were you the last generation not to get it or the first generation to get it? I think that is in the balance now.”

Miliband, who stood down as Labour leader after losing the election in May, said he would throw his energies into rallying support for a cross-party coalition – encompassing business, civic society and religious groups – that was capable of persuading the government to change direction on environmental policy.

“It is a very significant moment,” he said, “but it is a beginning not an end and the question now is can we build on this moment, on this opportunity, or not?”

The Conservative government has faced widespread criticism over a series of environmental decisions – on issues such as fracking, carbon capture and storage, the green investment bank and onshore wind – that scientists and campaigners say undermine its commitment to limit emissions and tackle climate change.

There was further anger last week after ministers backed plans to allow fracking under national parks and slashed support for the solar power industry just days after returning from Paris and at the same time as the Met Office predicted that 2016 would be the hottest year on record.

But Miliband said he remained optimistic that the government could be persuaded to change course and called for a domestic version of the “high-ambition coalition” that saw 100 countries working together for a stronger agreement in Paris.

“We need a high-ambition coalition … to bring together business, trade unions, civil society, religious leaders, scientists because all of them individually express real concern and interest and passion.

“The task is to pull this coalition together into a powerful, coherent voice because if there are pressures on George Osborne in the opposite direction we have got help people in the government who care about this issue so they can make the case that this is not the way to go.”

Miliband said the government’s record on the environment had been poor in the last seven months, describing it as a “bad move backwards”. But he was reluctant to make the government’s recent performance a party political issue, saying many “sensible Tories” understood what was at stake. “I think we can still build a cross-party consensus around this,” he said. “We have to.”

He said Osborne, the chancellor, too often gave the impression that the choice was between “a good economy or a good environment”, but he said most businesses understood that in future the two would go together.

“There are huge opportunities in the green economy and businesses need to know the direction of travel now so they can adapt and prosper,” he said.

Since Paris, Miliband has launched a campaign to push the government to become the first in the world to put a zero carbon emissions target into law.

But he said there was an ideological objection to tackling climate change in some sections of the Tory party that believed in strict free-market economics, viewing any state intervention with suspicion.

And he said some leading figures were more interested in positioning themselves for a future leadership bid than taking a long-term view on what was right for the country and the global environment.

However, he said the agreement in Paris, where almost 200 countries signed up to keep the world’s temperature rise under 2C, with an ambition to restrict the rise to 1.5C, was a big step forward and would “reverberate a long way into the future”.

Miliband said he was confident that he could work with people across the political spectrum to make sure the UK played its full part in the fight against climate change.

“We shouldn’t forget that the Climate Change Act in 2008 [which codified future emissions cuts in law] was passed with only three votes against,” he said. “It tells you something about where many people in the Tory party are. What I want to do is to start remaking that case on a cross-party basis.”