Britain's natural environment is likely to deteriorate after the United Kingdom leaves the European Union, according to a report commissioned by Friends of the Earth.

Its conclusions mean Prime Minister Theresa May's government is "unlikely" to deliver on its promise of a green Brexit, The Guardian newspaper reported on Thursday.

Kierra Box, a Friends of the Earth spokesperson, said the group hopes the government will now pay attention to protecting the environment after it is no longer bound by the EU's stringent environmental regulations.

"We were promised that Brexit wouldn't harm our environment－but this analysis shows that, under all scenarios currently on the table, this promise will be broken," Box said. "We hope this report will spur parliament to make much-needed changes to the withdrawal bill ... to lock in guarantees for our environment that the report authors have found lacking so far."

The damning report says wildlife, habitat, water quality, and air purity are all likely to deteriorate, and the control of chemicals and food safety regulations look set to be watered down.

Michael Gove, UK environment secretary, had said the country will be a global "champion" of green policies after it leaves the bloc at the end of next March.

Meanwhile, Michel Barnier, the EU's chief negotiator, seems unimpressed by the assurances. Reuters has reported he wants a "non-regression" clause to be included in any future trade deal between the UK and the EU, so the bloc's standards are preserved in the UK.

Barnier said May assured the EU that London will not water down its environmental protection, and he said her words were reflected in Britain's proposed 25-year plan for the environment.

"This is welcome, but my responsibility as the EU negotiator is to remain extremely vigilant," Reuters citied him as saying.

The report was written by academics at Sheffield University, Queen's University Belfast, and the University of East Anglia. They looked at likely gaps in environmental legislation, possible miscommunication between London and the devolved parliaments of Scotland, Wales and North Ireland, and other factors.

The researchers considered a range of Brexits, from a Norwegian-style arrangement that would keep the UK close to the EU, to a no-deal scenario and total separation. They found a "very high risk" to birds and habitats under all scenarios.