A move out of the EU would leave Britain’s environment open to the legislative whims of a Conservative government that has shown little inclination for good stewardship, according to a leading ‘green’ Tory.

If Britons vote to leave in the June referendum, many British environmental laws currently transposed from EU regulations would be open to the discretion of the UK parliament. Stanley Johnson, who held a European seat for the Tories during the 1980s, said the current government’s form was a poor recommendation for the repatriation of these powers.

“I’m a bit suspicious,” said Johnson, whose son Boris is a leading leave campaigner. “I’ve been a Conservative for years and years and years. But you know, you cannot honestly say that environmental policy has been top of the agenda of this Conservative government.

“I don’t think it has and I would be deeply worried to see a Treasury-led push to increase British competitiveness on the basis of a deterioration of environmental standards.”

On the contrary, said the pro-Brexit environment minister George Eustice, leaving the EU would allow British lawmakers to reform “clunky” environmental legislation.

“It would be pretty much the same, but you would have the flexibility to change things, to improve things, to make things better, to try new ideas,” he said.

Eustice’s government has consistently asserted that it has been the “greenest government ever”.

“I just think it’s a funny idea to get into which says that democratic government is a bad thing for the environment,” said Eustice, noting the large membership of the RSPB. “This country is passionate about animal welfare and wildlife. There will still be a very strong political lobby for high standards of environmental protection in this country.”

Johnson, who co-chairs the cross-party campaign group Environmentalists for Europe, said: “It would be nice to have that kind of trust in the British people ... National politicians are inevitably looking at short term issues. The uniqueness of the EU is that there is real, supranational power.

“Environmental policy is really too important to be left purely to local people or purely to national people. You actually have to have a global or international approach to some of the issues.”

Johnson turned up to meet the Guardian brandishing a copy of the EU’s 1992 habitats directive – legislation he helped to author and one of a tranche of “colossally important” laws he said would cease to govern Britain’s environment should voters decide to exit the EU.

Any Brexit scenario would mean the UK would leave the common agriculture and fisheries policies. It would also remove itself completely from habitat and water quality directives.

The Institute for European Environmental Policy (IEEP) report in March found that a complete departure from the EU “would create identifiable and substantial risks to future UK environmental ambition and outcomes”.

According to the IEEP, EU legislation stops states from watering down national standards in order to advantage local manufacturers. It also takes a long time to pass, but when it does it usually remains constant. This builds confidence in the industries that are affected and allows them to plan for sustainability.

Contrast this with the Conservative government’s mercurial green energy policies – described as “economic and climate lunacy” by the former energy secretary Ed Davey and decried as a “unholy mess” by advocates of clean and dirty energy alike.

The risk to the environment is lessened if a seceded UK remains within the European Economic Area (EEA) because many of the UK’s laws regulating the behaviour of industry are governed by the EEA’s free trade agreement. The country would continue to fund the EU budget, but would lose the opportunity to negotiate changes to laws, which are decided by the European parliament.

Implications of various scenarios related to the EU referendum. Photograph: IEEP

Despite strong social and environmental arguments for remaining within the union, Johnson said the remain campaign had fixated on the narrowest economic terms because “deep down Conservatives are interested in money”.

Eustice said no matter what trade arrangement was struck, Britain’s stewardship of nature would continue to be defined through the Bern Convention. But James Thornton, chief executive of the environmental law NGO ClientEarth, said the removal of EU law amounted to a significant stripping away of protection.

“International conventions are a type of law, but they are fairly soft. EU law is actually real law,” he said. “You need real and enforceable law because things that are voluntary or soft have never worked in the arena of the environment.”

Thornton said history showed that Britain – once known as the “dirty man of Europe” – had been a poor custodian of nature when left to its own devices. He cited Britain’s mid-20th century enthusiasm for overfishing as an example of the country’s exploitative zeal. “I suppose it could be enlightened now, but that wasn’t the experience in the past,” he said.

“Brexit is the most dangerous political threat to the environment that we are facing in the UK,” said Thornton. “Why are people saying we should leave the EU? Well because we should be free from regulations. It’s a very primitive, unsophisticated, uncivil way of thinking.”

In more recent years, he said, “in every case I’m aware of since this government has come in, the action has been contrary to environmental protection”. He said the UK’s failure to ameliorate breaches to air pollution limits – over which ClientEarth has successfully taken the government to court – was clear evidence of the need for EU oversight and the function of enforceable legislation.

“If you have a country in which the government is clearly happy to let 40,000 people a year die of air pollution, we need some external guidance to help them take care of their own citizens,” he said.

Responding to Eustice’s description of the continent’s regulatory regime as “clunky”, Thornton said that “in fact, the EU’s environmental laws are extremely well-designed” although they could be stronger.

In the key policy area of climate change, the chair of the UK’s Committee on Climate Change John Gummer, another leading Tory, said leaving would give away influence over continental and international ambition. The UK has lobbied the EU to increase its carbon cuts from 40% to 50% on 1990 levels by the year 2030.

“If we are not part of the encouragement for them to do it,” said Gummer. “Then not only will we not be helping the rest of Europe to do it, but we won’t do it ourselves.”

In international climate negotiations, the UK currently negotiates as part of the EU bloc, which was a leading force for strengthening the Paris agreement in December.

“The people who believe in Brexit do not accept a world in which you no longer have the sort of national power that we had in 1945 or 1900. You now operate within groupings. And if you are a powerful member of the most powerful grouping, you can get a great deal done,” said Gummer.