Environment secretary Michael Gove said the Office for Environmental Protection would hold the UK government to account and ensure targets on pollution, waste, water quality and wildlife protection were met.
But campaigners and policy experts said the proposed body’s powers were too “weak” for it to act as an effective safeguard after Brexit.
Friends of the Earth chief executive Craig Bennett said the draft plan “falls far short of the blueprint for a greener post-Brexit Britain.”
He added: “The much-vaunted green watchdog is too weak and fitted with blunt teeth.”
The Campaign to Protect Rural England (CPRE) and the Royal Society for the Protection of Birds (RSPB) also said the government’s current plan was inadequate and did not fulfil its promise to deliver a “green Brexit”.
Details revealed by Mr Gove’s department, suggested the Office for Environmental Protection would hold both government and public bodies to account.
It would be able to take legal action to enforce green laws where necessary, replacing the European Commission’s role after Brexit.
Draft clauses for a wider environment bill were also published. The bill would make it a legal requirement for government to have a plan for improving the environment, to be updated at least every five years.
“Our ambition is to be the first generation to leave the environment in a better state than that in which we found it,” said Mr Gove. “We will keep building on our successes by enhancing our environmental standards and delivering a green Brexit.”
But Tom Fyans, director of campaigns and policy for the CPRE, said: “Environmental principles are crucial to the way law is created, from planning and land use policy to air quality and biodiversity targets, yet the draft bill offers only the weak requirement that ministers “have regard to” or consider them.
“We are also seriously concerned that the (new) office will lack the true independence required to hold the government to account.”
The RSPB’s director of conservation, Martin Harper, said the plan for a green watchdog “falls short of the body we will need if we are to fill the gap in environmental protections left by Brexit.”
Mr Harper said: “As ministers scramble to prepare for a no-deal Brexit that would put our wildlife even further at risk, many of their key proposals, including a binding duty to secure the recovery of nature, remain in the form of policy promises, not law. Until this is remedied, the idea of a green Brexit will remain an elusive dream.”
Tom West, environment law and policy advisor for ClientEarth UK, added: “This bill is a step in the right direction and we are pleased that the government has listened to some of our concerns. But there is still a long way to go before the new watchdog has the strong legal teeth needed to protect our environment.”
The Independent has contacted the Department for the Environment for comment.