The mayor of Bristol has received a government warning to improve the city's air quality.
Marvin Rees has been told to complete his plans to reduce pollution, particularly from diesel engines.
He said he was looking at options but previously explained delays were caused over by concerns raised over proposals to introduce congestion charges.
It comes after the government itself got a High Court order to cut nitrogen dioxide in the UK down to legal levels.
Ministers said they wanted Bristol's plans handed in by the end of last year.
Therese Coffey, minister for the environment, said: "I think the challenge is that we have had very little from Bristol City Council at all.
"That's why we are encouraging them and why I've been public on saying they really need to get on with this."
In a letter to Labour's Mr Rees, Bristol's elected mayor, stated: "This means you have unlawfully failed to comply with the direction, and I am absolutely astonished at your delay in improving air quality for the people of Bristol."
The city council has said it is running technical modelling work to look at different ways of cutting pollution.
That includes looking at four different options for charging vehicles to come into Bristol.
In a previous interview with the BBC, Mr Rees had concerns about the impact of congestion charges for those on low incomes.
"What we don't want to do is have a disproportionate adverse affect on some of the poorest people in Bristol," he said.
"We have to look at the unintended consequences."
Green party city councillor, Jerome Thomas, said: "We believe that the experts have come up with the recommendations and that the mayor doesn't like them.
"As a result of that he's not revealing what those recommendations are, either to Bristolians or to the government."
A final plan must be delivered by 21 February and be put into action by the end of the year.