A third of Volkswagen cars fitted with software to cheat emissions tests remain unfixed, new figures show.
The rate at which vehicles are being dealt with has "stalled", according to Mary Creagh, chair of the Commons' Environmental Audit Committee.
She obtained figures from the Department for Transport which show that only around two-thirds of the 1.2 million vehicles affected in the UK had been fixed as of September.
Analysis shows that the rate of fixes has declined from a high of 10% of affected cars per month in February to just 2%.
Volkswagen sparked outrage in September 2015 when it was found to have fitted defeat device software to 11 million diesel vehicles worldwide.
Air pollution is linked to the early deaths of about 40,000 people a year in the UK, causing problems such as heart and lung diseases and asthma, and it affects children's development.
Ms Creagh has written to transport minister John Hayes to express the committee's concerns around Volkswagen's progress in removing the software.
She said: "It is over two years since the VW emissions scandal was discovered. A third of vehicles have yet to be fixed and rates have slowed considerably. We have written to the Department for Transport to ask what action they are taking in response to the stalled progress.
"It is essential that the vehicles on Britain's roads adhere to emissions regulations, particularly as the country is faced with dangerous levels of pollution.
"The Department must take responsibility for ensuring that these fixes are completed as soon as possible."
She also questioned why the rate of fixes for Skodas rose rapidly in July and August to roughly double that of other brands owned by Volkswagen.
Of the 1.2 million UK vehicles affected by the crisis, there were 508,276 Volkswagen cars, 393,450 Audis, 131,569 Skodas, 79,838 VW commercial vehicles and 76,773 Seats.
BBC One's Watchdog reported in July that some motorists experienced a drop in performance from their vehicles after being fixed, although Volkswagen insisted that "the overwhelming majority of our customers have been fully satisfied".
Demand for new diesel cars fell by 30% in October compared with the same month last year, according to figures published on Monday.
The Government recently announced plans to ban the sale of all conventional diesel and petrol cars by 2040 in a bid to meet European Union limits on harmful nitrogen dioxide pollution.
It is also considering funding measures to cut pollution with a tax on new diesel vehicles.
Volkswagen UK managing director Paul Willis told the Commons Transport Select Committee in February that the firm hoped to have completed the fixing programme by autumn of this year.
Greenpeace clean air campaigner Mel Evans said: "Time and again VW has dodged its responsibility.
"It's paid no fines, or faced criminal charges in the UK as it has in the US.
"Now it won't even meet its own timeline for a product recall of its dodgy diesels, which is currently the only redress on offer to the public here."