The details of more than 6000 drivers have been lost by Northern Ireland's Driver and Vehicle Agency (DVA), according to a report by the BBC.
The DVA is Northern Ireland's equivalent of the Driver and Vehicle Licencing Agency (DVLA).
The unencrypted data is said to include the drivers' names and addresses as well as information about their vehicles (such and make, model, chassis number and colour).
Two discs were sent by the DVA in Northern Ireland to the company's head office in Swansea, but didn't arrive.
'We are deeply disappointed and I can understand why customers would be concerned', said Brendan Magee, chief executive of the DVA.
The DVLA also confirmed that letters have been sent to all affected drivers.
Anyone concerned about the missing data can call a dedicated helpline on 028 703 4137 for more information.
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