David Cameron's speech on the future of the UK's relationship with the EU could prove to be a dark moment for this country's natural environment. Frustrated at their failure to cut what some ministers see as unnecessary environmental legislation through their red tape challenge (because many of the laws in question originate from Brussels), they now have an opportunity to turn their attention to what they see as the root of the problem through the opening that has emerged in the wake of prime minister's EU speech.

Cameron's carefully worded intervention expressed the view that "we need to examine whether the balance is right in so many areas where the European Union has legislated including on the environment." This was no throwaway line, neither was his demand that "nothing should be off the table". Some ministers see EU laws, including the Habitats directive and the Water Framework directive, as constraints to "growth" and believe they need to be weakened in order to promote economic activity.

But environmental policies and laws are essential for maintaining people's wellbeing, especially in a crowded country like ours. Anyone concerned about the state of the environment we all rely on should be truly alarmed by Cameron's intervention. More than 80% of our environmental legislation originates from the EU and it is very clear that plans to bring such laws under UK control are not for making them stronger. Recent remarks from George Osborne confirm the point.

In his 2011 autumn statement Osborne launched a bitter attack on the EU Habitats directive, claiming that it placed "ridiculous costs on British businesses". This legislation does a good job in protecting famous natural assets, such as the Cairngorm mountains, our finest chalk streams, the north Norfolk coast and the Dorset Heathlands, as well as a wide range of rare and vulnerable species, from eagles and dormice to dolphins and wild cats. He insisted that a review of the directive was undertaken to assess exactly how much economic damage it was doing. The result was evidently not what he expected.

The review Osborne demanded, and which was conducted by the Department for the Environment, Food and Rural Affairs, found that his hysterical claim was simply not true, and that in more than 99% of cases where a project was affected there was no objection from the statutory bodies concerned. And neither, according to this most comprehensive study ever done, is the UK's implementing "gold-plated" legislation. The deeply flawed thinking that led to the attack on the Habitats directive could not only lead to environmental harm but also serious economic damage. An example can be seen in the likely consequences of any attempt to weaken UK requirements under the Water Framework directive.

This legislation aims to protect and enhance the water environment, and thereby bring benefits not only for drinking water, fishing, wildlife and recreation, but also indirectly for flood risk reduction. It has demanding targets, but if we meet them we will be all the better off for having done so, including economically. Evidence to support that view comes from the government's own National Ecosystem assessment.

This comprehensive survey of the UK's natural environment estimated, for example, that the benefits derived from improved river water quality were about £1.1bn a year, while the amenity value of inland wetlands added a further £1.3bn. The numbers will be bigger still as the climate changes and flood risk increases. The idea that legislation to improve the water environment is an economic drain could not be more wrong. And then there is the impact on investor confidence.

The highly regulated private companies that supply water in England and Wales depend on capital being invested to pay for improvements in the infrastructure needed to meet our rising demand for clean water. These investors welcome clear regulation, because it gives certainty and reduces risk. By saying that environmental laws are on the table for renegotiation the relative certainty that investors until recently enjoyed has suddenly disappeared. This could jeopardise the many billions of pounds worth of investment needed in the years ahead to modernise our creaking water supply and sewage systems.

Precious natural places under threat of development, sewage-blighted beaches, declining salmon and trout populations, health-damaging air pollution – are these really the hallmarks of a modern competitive economy? While Europe is a complex equation, one thing is for sure: our lives are healthier and better as a result of the EU's high environmental standards.

Cameron's apparent contempt for environmental goals is hardly a natural Conservative agenda, or one that will win him much public support. He should remember that many of his own MPs were vocal in the face of plans to privatise our forests and strip back planning controls. There may be wind in the sails of the Eurosceptics now, but as a long-serving environmentalist I can only hope it is a gust that capsises them. If it does at least they'll be thankful the water's clean.

• Tony Juniper is the author of What has nature ever done for us?, published this month by Profile Books