The environment is still at risk of damage after Brexit, according to a cross-party group of 74 MPs and peers.
They say personal reassurances by Environment Secretary Michael Gove are of no value without new legislation.
Mr Gove has promised that the environment will be maintained or enhanced after the UK leaves the EU.
The group of 74 want a new Clean Air Act. They say they fear that, following Brexit, the government will pass the buck on pollution to councils.
They have outlined their concerns in a letter, which has been organised by Geraint Davies, chair of the All Party Parliamentary Group on Air Pollution.
He told BBC News: “Mr Gove talks the talk very impressively. But assurances are worth nothing until they are enacted.
“If there’s no deal, then the UK will be on a cliff edge in March. The government failed to meet existing standards, but it will no longer be able to be taken to court.
“It could easily pass the buck to local councils to transfer responsibility to them for tackling illegal air pollution.”
Mr Davies also re-iterated a demand from a joint committee of MPs reporting on air pollution. They said motor manufacturers who have cheated emissions tests should be forced to pay into a clean air fund.
“VW were fined heavily in the USA,” he said. “But here, Michael Gove is saying its better for them to invest the money designing better engines. That doesn’t wash with us.”
Mr Gove will reply to the letter shortly. But a spokesperson for his department said: “We will set out new primary legislation to secure a more coherent legislative framework for action to tackle air pollution.
“This will be underpinned by new England-wide powers to control major sources of air pollution plus new local powers to take action in areas with a problem.
“The government has launched a consultation on our proposals to establish a new, independent, statutory body to hold government to account on environmental standards once we have left the European Union, and on a new policy statement on environmental principles to apply following EU Exit.
“We are consulting as widely as possible in order to gather views on different models and options before making any final decisions.”
Mr Davies is unlikely to be assuaged by this response. He told us: “We have no idea how long Mr Gove will be in the department and we don’t know if he’ll get backing for legislation. We won’t believe it until we see it.”
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Brexit ‘could damage UK environment’