The United Nations has warned the government that Britain’s reputation is at risk over plans that would significantly weaken protections for the environment after Brexit.

In a stern intervention, Erik Solheim, executive director of the UN’s environment programme, called on the environment secretary Michael Gove to honour his promise to deliver a “green Brexit”, ensuring the environment would not suffer from Britain’s EU departure.

The warning comes after proposals to protect the climate after Brexit were dismissed as “toothless” by green campaigners. Under the plans, the new post-Brexit watchdog would not have the power to take the government to court over breaches of environmental standards. At the moment, the government is answerable at the European Court of Justice (ECJ), which often forces ministers to act.

Campaigners have warned that the current plans would leave Britain with a weaker system for enforcing environmental safeguards than those maintained in the United States by Donald Trump.

In an interview with the Observer, Solheim said it was “incredibly important that the UK keeps the environmental standards it has had under the European Union”.

“Michael Gove promised that would happen – that there would be no reduction of standards of any sort,” he said. “He even added that any change would be to better standards. There was a strong commitment to that from the government. Some of the opponents of the government had doubts about such statements, but that is the stated position from the government and it is a very good one. Any dilution and the UK reputation would be damaged. People in government need to make sure that does not happen. We need to make sure they have those standards or improve them, or meet the ones under the European Union.”

His intervention comes amid a Whitehall battle over the future of Britain’s environmental rules. While Gove has been pushing to give the new environmental watchdog all the powers currently enjoyed by the European commission, the Treasury is said to have resisted the idea because of its potential impact on post-Brexit growth.

Senior Tories have concerns about the weakness of the plans. Some Tory peers joined a Lords rebellion last week that attempted to force the government to keep all environmental protection in place under Britain’s EU membership.

As recently as last week, the UK and five other nations were referred to Europe’s highest court for failing to tackle illegal levels of air pollution. The ECJ has the power to impose large fines. Under current proposals, however, Britain’s new watchdog would have only the power to publish “advisory notices”.

Mary Creagh, the Labour chair of the Commons environmental audit committee, said the case proved that the new watchdog “needs enforcement powers not advisory notices”.

Mary Creagh, the Labour chair of the Commons environmental audit committee, said any new watchdog needed enforcement powers. Photograph: Lauren Hurley/PA

Solheim said: “The European Union has common standards all around Europe and of course the legal structure has contributed to that.

“These issues are up to the political conversation and for the people in the UK mobilised for environmental protections. We want to see a UK, if it is outside the EU, that can even go further in that direction.”

He called on Britain to keep working with the EU on green issues after Brexit. “We need global leadership. Europe and China are the most likely sources for that leadership, so working together on this will be very important,” he said. “The UK has historically had a global perspective as a result of its colonial heritage. The global perspective is stronger in London and Paris than any other capital. So it is very important that the UK continues to be engaged.“We are about to start negotiating a global pact for the environment, which has been initiated by President Macron [of France]. There should be no sliding back on the progress.”

John Sauven, executive director of Greenpeace UK, said: “Michael Gove promised a green Brexit, but as things stand his toothless watchdog would leave us with a weaker enforcement agency than Trump’s America. This intervention from the UN environment chief is a sign of growing concern and shows the world is watching. Nobody voted for a dirty Brexit that leaves our beaches, water and air quality worse off.”

James Cameron, the climate change investment entrepreneur and a former member of David Cameron’s business advisory group, said that legal powers were essential, adding: “European law has improved air and water quality, our beaches and natural habitats.”

However, Gove insisted that Brexit would not put the environment at risk. “We will not weaken environmental protections when we leave the EU,” he said. “A new environmental principles and governance bill will ensure core environmental principles remain central to government policy and decision-making. This will help us to deliver a green Brexit and the vision set out in our 25-year environment plan.

“But we will only achieve our aims by also creating a strong and objective voice that champions and enforces environmental standards. That’s why our bill will also create an independent and statutory watchdog. This will hold governments to account for delivering their commitments to the natural world.”