The UK's environmental policies face a "cocktail of threats" from Brexit, it has been warned.
More than 1,100 European Union environmental laws, ranging from air pollution limits to energy efficiency and wildlife protection, need to be transposed into UK law, Green Party MP Caroline Lucas said.
But many protections could be downgraded as the UK leaves the EU, while desperation for a trade deal with the US could open domestic markets to lower standards such as chickens washed in chlorine and beef treated with hormones, she said.
Environmental laws could be unenforceable as the UK has no system to oversee compliance, which is currently done by European bodies, while the country would also be leaving key EU agencies which support and develop green policies.
The environment could see a reduction in funding as EU cash to support nature protection schemes and wildlife-friendly farming ends and the UK could quit key schemes such as the emissions trading scheme which aims to drive down carbon pollution.
A report by Ms Lucas called for a "green guarantee" to ensure the environment is protected, including a new Environmental Protection Act to re-establish protections that could be lost or rendered meaningless in the transfer of regulations into UK law.
Ms Lucas, a member of the Environmental Audit Committee, said: "Though we've hardly heard it mentioned by the Government, it's clear that British environmental policy faces a cocktail of threats from Brexit.
"Just days after the Brexit vote in the Commons we can clearly see the huge risks of downgrading environmental protections as part of the post-referendum process.
"Key laws could become unenforceable, spending on crucial schemes could be cut and new trade deals could undermine existing regulations."
She also warned of a much greater probability of changes to UK environment laws outside the EU, more exposure to political cycles and a danger that investors in clean technology will be wary of potentially higher risks.
"We need a Green Guarantee that will deliver on Government's commitment to ensuring that "we become the first generation to leave the environment in a better state than we found it.
"We need to immediately begin work on introducing an Environment Act to ensure that Britain crucial rules and enforcement don't drop off as Britain exits the EU," she urged.