Former Labour leader Ed Miliband has warned an EU exit would be bad for wildlife and the environment.
Environment Secretary Liz Truss, Lib Dem ex-energy secretary Sir Ed Davey and Green Party MP Caroline Lucas also signed a declaration saying the EU was "central" to tackling climate change.
But pro-Leave Marine Environment Minister George Eustice said quitting the EU would give the UK a stronger voice internationally.
The UK's EU referendum is on 23 June.
The campaign to stay in the EU described the pamphlet signed by the four politicians as an "unprecedented partnership".
It accuses Leave campaigners of a "cavalier ignorance" about the environment.
The document has moved here.
The "added clout" of other EU members allows issues such as illegally logged timber and commercial whaling to be tackled, they say, adding that the "global environment" will be under threat if the UK votes to leave.
EU membership also supports jobs in the "green economy" in the UK, they said.
Labour MP and Vote Leave co-chairwoman Gisela Stuart said the pro-EU campaign was "descending into absurdity".
"It seems there is no good in the world that they cannot somehow attribute to the EU and no imagined disaster they cannot predict if we vote leave," she said.
Mr Eustice said the EU had "systematically undermined the UK's place on international wildlife conventions" and said the country could "regain its voice" by leaving.