The minister who knew that millions of drivers' records had gone missing in the US said he assumed his successor would hear about a probe into the loss.
Mr Ladyman had left office before the investigation had finished.
Stephen Ladyman was transport minister in May when the details, including names, addresses and phone numbers on a computer hard drive disappeared.
Asked why the public was only told on Monday, he said: "I assumed the new minister would've been told about it.
"It doesn't look like a cover up to me - it looks like one of those things."
Mr Ladyman spoke out to defend his role a day after Transport Secretary Ruth Kelly appeared before the Commons to reveal that the details of three million candidates for the driving theory test had gone missing.
It was the second time in a month that a minister has issued such an apology, following the loss by HM Revenue and Customs of discs containing 25m people's details.
Ms Kelly said the details of learner drivers had been formatted specifically to meet the security requirements of the private contractor, Pearson Driving Assessments Ltd, and were not "readily usable or accessible" by third parties.
She said the details were not sent in the post, but the hard drive had not been found where it had been expected to be, in a "secure facility" in Iowa.
Mr Ladyman said he was told about the problem by the chief executive of the Driving Standards Agency when he was a junior transport minister in the summer and immediately ordered an investigation.
He was sacked by Gordon Brown when he reshuffled the government after succeeding Tony Blair, and said he expected the civil service to inform his successor of the probe's outcome.
"I was told that Pearson, the company involved in Iowa, had reported to the DSA that a disc had been lost inside their secure facility," he told BBC Radio 4's Today programme.
PEARSON DRIVING ASSESSMENTS
A company registered in England and Wales
Its trading name is Pearson Vue
It is part of the London-based Pearson, an international media company
Pearson owns the Financial Times and Penguin books
Contractor for testing learner drivers for UK's Driving Standards Agency
Driver records sent to Pearson data center in Iowa City
Pearson founded by Samuel Pearson in 1844, beginning as a small building firm in Yorkshire
Pearson Education created in 1998
"In all likelihood it was a simple case of data room mismanagement, but they weren't taking any chances, so they had reported the matter to the DSA.
"I, of course, realising the significance of it, if the data had got into the wrong hands, ordered a thorough investigation, a review of their procedures and took advice as to whether it needed to be made public.
"Given that it looked in no possible way that this data had got into the wrong hands, the sensible thing to do was to wait until we had the results of the investigation.
"This was immediately before the reshuffle and before they could come back to me and tell me the results of their investigation, the reshuffle had taken place and I left office.
"My assumption was that they would come back and tell the minister who replaced me."
Asked if the hard drive had since been found, he said: "I have no idea because I left office before the investigations could come back to me. That's a matter that Pearsons will have to tell us."
Pearson, the company which owns the Financial Times newspaper and Penguin books, won a contract for testing learner drivers for the UK's Driving Standards Agency in 2003.
It reported the loss to Iowa City police on Monday - about seven months after it had first known.
Police said it was "unlikely" the drive would be found.
Iowa City police sergeant Troy Kelsey said an internal search had been carried out by the company initially.
"They scoured the premises. They do not believe that there was any criminal intent, if you will," he told BBC Radio 5 Live.
'No need for notification'
"There really wasn't any evidence that it had been stolen, but they could not locate the physical hard drive on their property and they were wanting to report it."
He said there was no evidence that the data had been misused, but added he was not confident the drive would be found.
"At this point in time, enough time has gone by that's probably unlikely, but one never knows," he said.
Shadow transport secretary Theresa Villiers said: "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."
Phil Booth, national coordinator of civil liberties and privacy campaign NO2ID, called for an immediate freeze on all mass transfers of private information between government agencies - and for the scrapping of plans for ID cards.
"For all its spin about tackling identity fraud, the government is clearly the largest contributor to the problem," he said. "These abuses must stop now."
Unlike the lost child benefit data, the learner driver details did not include bank account details, National Insurance numbers, driving licence numbers or dates of birth, Ms Kelly said.
The Driving Standards Agency has set up a dedicated advice line for candidates who took their driving theory test between September 2004 and April 2007.
Public and Commercial Services Union general secretary Mark Serwotka said: "This latest data loss by a private contractor in the US raises serious questions about the role of the private sector and the security of data."