Kwik Fit has confirmed that its computer network has been infected with malware, disrupting its ability to book in vehicle repairs and handle other customer requests.
It has since told the BBC that it is still experiencing some problems.
However, it declined to elaborate.
"We first experienced some issues with a virus in our IT network during the weekend," said a spokeswoman.
"This affected a number of our systems but in the interest of ongoing security we can't confirm the source of the problem.
"We have been working to get our operational systems back up and running normally and while there is still some disruption, our centres are open as usual."
Several customers have sent messages to the firm via social media to express frustration.
@Kwik_Fit you cancelled my tyre fitting appointment on Saturday, promised that you would arrive to fit my tyres this morning (Monday)...... no show and no contact.— Jase O'T (@Jase_OT) January 28, 2019
“You can’t get better than a Kwik-Fit Fitter, were the boys to trust” ......
We are sorry to hear this. This could be down to the fact that we are having some technical issues at the moment. Are you able to call our customer service team on- 0800 757 677. Thank you. -BH— Kwik Fit CS (@KwikFitCS) January 28, 2019
@Kwik_Fit Turned up at Kwik Fit Coventry at 10am for a booked brake test and was told that the systems have been down for more than 24 hours so they can't carry out the test. Gave my number and told them to call me when the systems have been sorted. Heard nothing. any update?— Steve Rumney (@STEVEN_RUMNEY) January 27, 2019
The company declined to discuss what the motivation for the attack might be, but said that it did not believe its customers' records had been affected.
"We can reassure customers that we do not store any of their financial information and currently have no reason to believe that any customer data was compromised," said a spokeswoman.
"For security reasons we are not able to say anything else."