Ed Miliband has joined forces with the Conservatives as part of a cross-party push to persuade voters that leaving the European Union would damage the UK's environment.
The former Labour leader has signed a joint declaration with Environment Secretary Liz Truss, Liberal Democrat ex-energy secretary Ed Davey and Green Party MP Caroline Lucas in what the pro-EU called an unprecedented partnership.
The quartet portrayed those campaigning for a Brexit as "extreme and outdated" climate change deniers who showed a "cavalier ignorance" about the potential impact.
"Collective action is the only solution to rising seas and rising temperatures. The European Union is central to both these challenges," they wrote in a pamphlet setting out the arguments.
EU membership supported domestic action to improve air quality, protect nature and wildlife and invest in renewable energy, it suggested, and Brussels was "a leader in the battle to secure binding agreements" internationally.
"Those campaigning for Britain to leave Europe cannot be trusted on the environment," they said.
"They have opposed vital green measures and denounced climate change as 'mumbo jumbo'. They demonstrate a cavalier ignorance about climate matters which embodies the extreme and out-dated outlook of those who want to leave.
"If Britain leaves Europe, our environment, our wildlife and our global habitat will be starved of investment, bereft of protections and denied the leadership it needs.
"With the added clout of 27 other member states we can tackle environmental problems like preventing trade in illegally logged timber, and preventing commercial whaling, and cracking down on the black market trade in ivory and rhino horn.
"Out of Europe, our legacy to future generations will not just be a Britain that is weaker and worse off, but a global environment that is under threat."
Marine environment minister George Eustice - who supports Vote Leave - said the EU had "systematically undermined the UK's place on international wildlife conventions.
"We have already been stripped of our voting rights on regional fisheries management organisations and, extraordinarily, it is now unlawful for the UK to speak at wildlife conventions like Cites (the Convention on International Trade in Endangered Species of Wild Fauna and Flora) without first getting permission for what we want to say from the European Commission.
"If we vote to leave and take control, the UK would regain its own seat and its voice in vital international wildlife conventions and everything from promoting shark conservation to ending whaling would become much easier."