Around 34,000 vehicle owners have had their NCT certificates revoked due to a 'system error'.
The National Car Testing Service says some drivers were wrongly provided with a two-year cert instead of a one-year cert.
The certs in question were issued incorrectly in the last 12 months.
It only relates to cars that were over 10 years old at the time of their last test and which had their NCT extended by four months due to the pandemic.
These vehicles were given a cert valid for two years instead of one.
The incorrect certificates, issued at the time of the inspections, have been revoked and new NCT certs are being issued to all affected customers.
In a statement, the NCT service says this should be complete by August.
Anyone who has purchased one of the affected vehicles with the expectation of a longer NCT certificate duration will be offered a full free test if the vehicle is booked in advance of its revised test due date.
In a statement, the NCTS said: "We would like to reassure customers that NCTS has taken all the necessary steps to correct this administrative error and that we will be in contact directly with each affected customer.
"There is no need for you to take any action until you hear from us.
"Members of the public, looking to purchase a vehicle over 10 years old, are advised to check the vehicle’s NCT certificate expiry date, before purchase."
Lockdowns have seen disruption to the NCT testing system, with many certs extended due to the restrictions.
