The Driver and Vehicle Licensing Agency (DVLA) has admitted it sent confidential details to the wrong motorists by mistake.
The Driver and Vehicle Licensing Agency building in Swansea
The Swansea-based agency has confirmed at least 100 sensitive documents were sent to the wrong addresses.
Earlier this week it sent out 1,215 questionnaires including drivers' names, addresses, birth dates, licence numbers and motoring offences records.
The agency said at least 100 people who have been affected have got in touch.
Officials said they did not know exactly how many people had been affected but a dedicated telephone line had been set up.
The DVLA admitted that "human error" had led to the "isolated incident".
It said all 1,215 drivers who were sent the questionnaires were being contacted to advise them of the error, apologise and discuss its implications.
The drivers had been contacted as part of a routine DVLA survey, it said.
The agency said notes had been placed on all 1,215 records on its database to prevent any fraudulent activity being attempted on that record.
It said it had sought advice from the police regarding any risk of identify theft to customers and had also contacted the Information Commissioner.
DVLA chief executive Noel Shanahan apologised and said it was in the process of contacting customers who have been affected.
"We have initiated an internal inquiry to establish exactly how this error occurred and to identify any changes in procedures that may be necessary," he said.
100 million letters
Mr Shanahan said that the error first came to light on Wednesday when the DVLA received calls from customers who had received envelopes in error.
Some of the 1,215 customers had the right information but others had their name and address on the front but the details of others inside, he said.
"We don't yet know exactly how many have got the wrong information but we know that some of them have," he said.
"This was definitely a human error.
"About 100 million letters leave the DVLA every year. We have a very structured process and what happened in this isolated incident is that we didn't follow that particular process and we are now investigating as to why."
Conservative MP Nigel Evans, chair of the all party group looking at identity fraud, said he was concerned but assured by the DVLA's actions over the matter.
"I hope the DVLA will now think long and hard about the actions that they need to take to ensure this doesn't happen again," he said.
He questioned whether the error would have come to light if it had not emerged last month that personal information on 25m people on two CDs had been lost by HM Revenue and Customs.
"What I am now thinking is that there must be a number of other government departments that need to look at their processes and errors that have been made in the past," said Mr Evans.
Anyone who fears they may have received the wrong details from the DVLA should call them on 0800 085 2333.