More than one in 10 Volkswagen customers who had their cars fixed following the diesel emissions scandal have since lost power at high speed, a law firm said.
Several motorists feared for their lives after being forced to swerve across two lanes of motorway traffic when the incidents occurred, according to Slater and Gordon.
The firm surveyed the 40,000 car owners it represents who are seeking compensation over the use of software to cheat pollution tests.
Semina Mohammed, 37, was driving her VW Golf TDI on the M56 near Chester a few days after undergoing the fix when the car juddered to a halt.
The secondary school teacher, from Altrincham, Cheshire, said: “I’ve never been that frightened in the car before. I genuinely feared for my life.
“I was in the outside lane, overtaking another car, and suddenly I felt the power dropping.
“I managed to get across. If there had been a car next to me I wouldn’t have been able to.”
The survey also found that almost a fifth (18%) of drivers whose cars have undergone the fix have suffered reduced fuel efficiency, and 8% have had more than five separate malfunctions.
Slater and Gordon lawyer Gareth Pope said: “It has been over two years since the scandal was exposed and UK consumers have waited in vain for Volkswagen to respond to their complaints fairly.
“VW’s only response has been to offer consumers in the UK a fix that our clients are telling us doesn’t work.
“The survey results are deeply troubling.”
Concerns over nitrogen dioxide emissions have grown since Volkswagen was found in September 2015 to have cheated air pollution tests for 11 million diesel vehicles worldwide, including 1.2 million in the UK.
Owners of affected vehicles in the US are being offered compensation worth thousands of pounds each, but the manufacturer is refusing to make payouts in the UK.
The next hearing in a UK civil claim against the German firm takes place at the Royal Courts of Justice on Tuesday.
A Volkswagen Group UK spokesman said: “There is no legal basis for customer claims in connection with the diesel matter.
“Our UK customers have not suffered any loss or damage as a result of the NOx issue.
“The vehicles are safe and roadworthy, and perform as advertised.”
The voluntary fix has been implemented on around 840,000 affected UK vehicles, with the “overwhelming majority of customers in question fully satisfied”, the spokesman added.