The UK’s new green watchdog will be almost powerless to impose penalties on public authorities that commit environmental damage and will lack independence from ministerial interference, a top law body has warned.

The Office for Environmental Protection, which replaces the European Commission as the domestic enforcer of environmental regulation after Brexit, will be weaker and less effective than its EU predecessor, according to environmental groups.

The Bingham Centre for the Rule of Law said the government’s plans for the new body, which are featured in the long-awaited environment bill, would “undermine the rule of law” because they stopped courts from imposing penalties for illegal pollution and other breaches in most cases.

According to the proposals, if a UK court found that a public authority had acted illegally by, for example, approving a development, it would have no power to reverse the act if doing so caused “substantial hardship” to another body, such as the developer.

“This means that a clear unlawful action must be allowed to continue if a third party stands to benefit from it,” said the Bingham Centre’s analysis.

The environment secretary will also be able to give the OEP guidance on its enforcement work, which the watchdog must “have regard” for. Ronan Cormacain, senior research fellow at the Bingham Centre, said that was “a pretty clear control or restriction on the exercise of independent thought by the OEP”.

Hatti Owens, a lawyer at environmental law group ClientEarth, said the proposals would “render the court’s decisions [on breaches of law] almost entirely useless”.

The limitations imposed on the watchdog renege on promises made in 2017 by Michael Gove, then environment secretary, to create an independent body that would “hold the powerful to account” after Brexit, said environmental groups.

Ruth Chambers, senior parliamentary affairs associate at think-tank Green Alliance, said the government had given the environment secretary “the power to tell the OEP how to do its job”, and placed “in our experience [an] unprecedented limit on the court’s ability to grant a remedy”.

Ahead of hosting November’s international climate summit, known as COP26, the government has sought to present itself as a leader on green issues.

However, the environment bill, first published in 2018 and debated in the House of Lords this week, has come under repeated criticism. A group of cross-party MPs warned in 2019 that the OEP would be “fragile”, lacking in independence and unable to scrutinise the government properly.

In the Lords’ debate on Monday, a series of speakers called for the OEP to be granted more independence.

“Where we needed a powerful, independent OEP, backed up by the full force of the law, the bill gives us a hobbled regulator, its independence compromised by the ability of ministers to interfere in how it carries out its enforcement functions,” said Liberal Democrat peer Lord Jonny Oates.

Cross-party peer Baroness Rosie Boycott said if the government took the environment “as seriously as they say they do, I do not understand at all why this cannot be an independent body”.

The Bingham Centre said changes should be made so that acts ruled as unlawful are “presumed void” and the court should “retain its usual discretion to grant the remedy it deems appropriate, without the unnecessary restrictions introduced by the bill”.

Defra said the environment bill would “ensure the independence of the OEP, so the new body will have the power to scrutinise environmental policy and law, investigate complaints and take enforcement action”.

“The environment secretary will not be able to intervene in decision making about specific or individual cases,” it added.