Only around one in 10 Volkswagen vehicles fitted with software to cheat emissions tests were fixed in the UK in the year since the scandal broke, figures released by the manufacturer show.
The controversy began on September 18 last year when US regulators told VW to recall 482,000 diesel cars after discovering they contained illegal defeat devices.
The Environmental Protection Agency said the software allowed cars to release fewer smog-causing pollutants during tests than in real-world driving conditions.
Volkswagen said 1.2 million vehicles were affected by the issue in the UK, which was equivalent to nearly one in 10 of the country's diesel cars.
This includes the VW brand, Audi, Skoda, Seat and VW commercial vehicles.
Volkswagen told the Press Association that "over 110,000 vehicles in the UK" have undergone remedial action - a figure which Louise Ellman, Labour MP and chair of the Transport Select Committee, described as 'simply unacceptable'.
She said: "One year on from the Volkswagen emissions scandal, nine out of 10 drivers are still waiting for their car to be recalled. Time and time again, VW's schedule has slipped.
"People deserve to know when they can expect their vehicles to be corrected and returned to them. It's time VW came clean with its customers. If it refuses to do so, the Government must act."
Volkswagen said the process of having the fixes approved for different models by regulators in various countries is complex.
It added that it has written to more than 300,000 UK customers requesting them to have the modification work carried out.
Jim Holder, editorial director of magazines Autocar and What Car?, said the fixes were taking "far longer to be approved than VW's bosses anticipated".
As a result of the scandal Martin Winterkorn resigned as VW's chief executive . He was replaced by former Porsche boss Matthias Mueller.
In June Volkswagen agreed to settle consumer lawsuits and government allegations in the US by taking steps that could cost the manufacturer US$14.7 billion ($20.1 billion).