Environmental groups have urged the prime minister to avoid watering down legislation on climate change and wildlife protection after Brexit.
The letter, signed by WWF, Greenpeace and others, says action is needed to halt ongoing environmental decline.
The UK government previously said it would leave the environment in a better state for future generations.
But the letter expresses concern that Brexit could force ministers to backtrack on this promise.
Number 10 said it would study the letter before responding.
The letter says: “We are alarmed by recent media reports suggesting that the UK’s commitments to tackling climate change and ending the illegal wildlife trade could be watered down to secure post-Brexit trade deals.”
It added: “To be a great, global trading nation, the UK must deliver on its promises for the environment and the climate and honour our international commitments.
“In doing so we will help build a greener, better and more prosperous future for everyone, rather than driving an environmental race to the bottom.”
Campaign groups that have signed the letter include WWF, the Born Free Foundation, Cafod, Friends of the Earth, Greenpeace and Oxfam.
A number of high-profile individuals are also signatories: they include Andy Murray, Anna Friel, Will Young, Lord Stuart Rose and environmentalist Sir Crispin Tickell.
“A majority of environmental protections derive from the EU – we’ve probably got the world’s leading environmental framework. So as the UK leaves the EU, the risk is that some of those protections don’t get brought over into UK law,” Trevor Hutchings, director of advocacy at WWF-UK, told BBC News.
These protections include a package of legislation to ensure the EU meets climate and energy targets for 2020. He said the EU institutions also provided a strict system of accountability, so that fines could be invoked if member states did not deliver on their obligations to the environment.
“We, on the one hand, welcome the government’s commitment to nature and to bringing these laws over through the Great Repeal Bill… but in recent weeks a number of things have happened that question that commitment,” Mr Hutchings explained.
He said this included not only the leaked memo, but also the delayed release of the government’s 25-year plan for nature in England and the Clean Growth Plan on reducing carbon emissions, which Mr Hutchings said was expected before March this year.
“Things like access to environmental information, the ‘precautionary principle’, the ‘polluter pays’ principle are all established in EU treaties. The sense is that those might not come across [into UK law],” he explained.
Spokespeople for Number 10 and the Department for Environment, Food and Rural Affairs (Defra) said officials would look at the detail of the letter before issuing a response.
But sources said the government’s commitment to green issues remained as strong as ever.
Nevertheless, some observers within the environmental community have expressed concerns that green issues could become bargaining chips in the effort to secure trade deals.
“Environmental regulations and standards do make up so-called non-tariff trade barriers – as some people might perceive them,” said Mr Hutchings.
“It probably doesn’t come as a surprise that some of these non-tariff barriers could become a central negotiating point for trade deals with countries that have lower standards. That’s the fear, though we don’t have any clear evidence of that.”
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