Five leading green organisations have called on Theresa May to delay Brexit to avoid losing environmental protections created by “decades of campaigning”.

Friends of the Earth, Greenpeace UK, Green Alliance, E3G and ChemTrust have said the prime minister should try to extend article 50 because the government has failed to live up to its promise that existing environmental standards would be maintained or even enhanced.

The call for a delay comes after Greener UK, which represents 14 environmental groups including WWF, ClientEarth, the CPRE, National Trust and the RSPB, reported there was a “high risk” that all areas of the environment – air pollution, fisheries, chemicals, climate change, water, farming, waste and conservation – would be less protected after Brexit.

This is the first time many of these areas have been deemed to be at high risk by the Greener UK coalition, which has been assessing the government’s Brexit progress with its Risk Tracker analysis.

Craig Bennett, the chief executive of Friends of the Earth, said even if May’s withdrawal agreement was signed, there would already be “big, big gaps in areas of protection”. “This time last year, Theresa May solemnly promised – and Michael Gove has solemnly promised – to maintain and enhance environmental protections after Brexit. But the way things stand, with just a few days to go, they are going to clearly fail on delivering that promise.”

Even if there is a deal, Bennett said, attempts to transfer EU regulations into law by using statutory instruments had already weakened environmental rules. He described plans for the Health and Safety Executive to take over chemical regulation from the European Chemicals Agency as “hopeless”.

He said: “There’s no way it’s going to be able to do the same job with the resources its got. We’ve run out of time.

“Although the country’s very divided at the moment, the one thing that unites us, whether you voted leave or remain, is that no one voted for the environment to be worse off as a result of Brexit. We need to extend article 50 to at least give the extra time in place to put these measures before we Brexit.”

Bennett said if there was no deal, there would be no time to implement measures such as the green watchdog that the environment secretary Michael Gove has promised will be able to sue the government for environmental breaches.

“All the environmental groups are really concerned that if the economy suffers, it’s going to push the environment even further down the list of priorities,” he said. “Promises we’ve had from Michael Gove will get traded away in a bid to sign quick trade deals. Just this week, we’ve seen the evidence of how the US is going to push very hard for us to lower our environmental standards.”

Shane Tomlinson, deputy chief executive of E3G, said leaving the EU on 29 March would have “dire consequences for our efforts to tackle climate change, creating massive uncertainty for low-carbon investments and undermining our efforts to decarbonise the economy”.

“If choosing between a no deal or extension to the process next week, politicians who care about our environment must back extending Article 50,” he said.