The Government has lost the records of more than three million learner drivers from a "secure facility" in Iowa in the United States, Transport Secretary Ruth Kelly has admitted.
She told the Commons the latest security breach did not include personal data such as credit card or bank details.
But it is a huge embarrassment to the Government, coming on the same day Chancellor Alistair Darling said Ministers still did not know the whereabouts of two missing computer discs containing sensitive information on 25 million people.
Mr Darling gave a non-committal end-of-term update to MPs on the Poynter Review into the missing child benefit data-base, saying it would be next year before full conclusions would be known.
Ms Kelly was then forced to concede to MPs: "In May this year, Pearson Driving Assessments Ltd, a private contractor to the Driving Standards Agency, informed the agency that a hard disk drive had gone missing from its secure facility in Iowa City, Iowa.
"The hard disk drive contained the records of just over three million candidates for the driving theory test."
The records contained the L-driver's name, postal address, phone number, the test fee paid, their test centre, a code indicating how the test was paid for and an e-mail address.
But Ms Kelly stressed "The hard disk did not contain details of any individual's bank account or credit card.
"It did not contain their driving licence number, nor their National Insurance number.
"It did not contain their date of birth, nor a copy of their signature and it did not contain the result of their test."
The Transport Secretary also said the disk was "formatted specifically to fit Pearson configuration" and was not easily read by third parties.
Because banking details were not included in the lost data, individuals are not being informed, she said.
The department was taking steps increasingly to use electronic transfer of data to guard against future risks, added Ms Kelly.
Her admission came after Mr Darling told MPs there was little progress in the investigation into the loss of the two child benefit discs, despite widespread police searches and the offer of a £20,000 reward for their return.
The Chancellor said: "I can confirm that the police investigation continues. And while the searches are drawing to a close their inquiry is not yet complete.
"However, the police have re-iterated that they have no intelligence that the data has fallen into the wrong hands. They will keep that under review."
He said banks had also found "no evidence of any activities suggesting evidence of fraud arising from this incident".
Mr Darling said PricewaterhouseCoopers chairman Kieran Poynter, appointed to lead the investigation into the incident "says his work is far from complete and his conclusions will develop as his work progresses". He will complete his work next year.
Shadow Treasury minister Philip Hammond said it was "a wholly inadequate response from a wholly inadequate Chancellor".
And shadow Transport Secretary Theresa Villiers said: "This latest debacle shows a systemic failure within the Government's data protection controls. The words Labour and incompetence are now synonymous.
"The importance of confidentiality for the DFT's agencies is particularly acute given the value of vehicle and driver details to car criminals. This latest failure must be another nail in the coffin for Ruth Kelly's flagship transport policy - an untried and untested national road pricing scheme.
"How can the public possibly trust this incompetent department with information on every journey made by the 33 million vehicles on Britain's roads?"