Volkswagen said it has so far fixed fewer than half of the 1.2 million cars affected by the diesel emissions scandal in Britain, 18 months after the revelations first came to light.
The German carmaker admitted in September 2015 to using software to cheat diesel emission tests in the United States and has since paid out compensation to U.S. motorists but has refused to do so in Europe.
In Britain, Europe's second biggest autos market where Volkswagen Group is the top seller, the firm has faced pressure from lawmakers who have repeatedly questioned the brand's managing director.
In a response to lawmakers' latest letter, VW's Paul Willis said the firm was nearly half way to fixing all models.
"We have implemented the technical measures in more than 540,000 UK vehicles," Willis told lawmakers in a letter dated Mar. 24, which was released on Friday. In February, he said the total stood at 470,000.
VW has not set a firm deadline to complete the work but hopes to have most of it done by the autumn.
Willis also denied that any of the changes made had negatively affected the performance of vehicles, an issue at the heart of attempts by some law firms to take legal action against the company.
"The technical measures have been rigorously tested and the relevant authorities have confirmed that there is no adverse impact on the vehicles' MPG, CO2 emissions, engine output, maximum torque and noise emissions" he said.
(Reporting by Costas Pitas; Editing by Keith Weir)