Only around one in 10 UK Volkswagen vehicles fitted with software to cheat emissions tests were fixed 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.
"This is simply unacceptable when Paul Willis, VW UK's managing director, promised the Transport Committee that VW drivers could expect to have their cars fixed by the end of this year.
"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".
He went on: "We know from the emails we've received from angry owners that this has left many of them frustrated - not just because there is no end to the saga in sight, but also because they don't feel VW has communicated well with them."
Sales of Volkswagen cars in the UK have fallen following the scandal.
Figures from the Society of Motor Manufacturers and Traders show that registrations in the first eight months of the year were down by 10% compared to the same period in 2015.
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 14.7 billion US dollars (£11.2 billion).
The company has been criticised over its decision to compensate customers in the US with up to 10,000 US dollars (£7,600) but not give anything to UK owners.
Alex Neill, director of policy and campaigns at consumer group Which?, said: "One year on and VW customers in the UK will be questioning why US consumers are getting compensation while nothing is on the table for the 1.2 million owners affected in this country.
"The Government has had a year to address this issue, they now need to urgently ensure that UK customers are treated fairly."
Law firm Leigh Day represents around 10,000 Volkswagen owners and is preparing a group legal action against the manufacturer.
Shazia Yamin, a lawyer at the firm, said: "One of the reasons given by Volkswagen in refusing to compensate UK owners is that these vehicles will be repaired with no adverse effect on the vehicles' performance.
"However, to date less than 10% of the affected vehicles have been repaired. More worryingly, Volkswagen have released no detailed information on what the repair involves, leaving affected owners in the dark about what is actually being done to their vehicle."
A spokesman for Volkswagen UK said: "At Volkswagen Group UK customers are our priority and every owner has been written to at least three times to keep them up to date.
"We are working hard to apply the approved technical measures to as many cars as swiftly as possible as soon as they are approved.
"The process of applying the technical measures has been under way since January 2016 and we have applied the measures to over 110,000 vehicles in the UK.
"We will continue to work closely with the authorities involved to fulfil our commitments."
A Department for Transport spokesman said: "The Government takes the unacceptable actions of VW extremely seriously. That is why we have taken swift action to protect the UK consumer.
"We were the first country in Europe to complete our own tests (April 2016) to ensure the issue was not industry-wide.
"We continue to push VW to take action to compensate the UK consumer.
"Last week the new Minister of State met with the MD for VW UK, Paul Willis, to reaffirm the Government's calls for compensation and press for a fix to affected cars to be carried out as quickly as possible."