The details of more than 1000 British school pupils were lost when a USB stick was misplaced.
According to deadlinenews.co.uk, the records were downloaded onto a private memory stick before being taken home and lost by a council worker.
Records included names, ages, emergency contacts and some medical history of 1075 pupils at primary schools across East Lothian.
Council education chief Don Ledingham said the incident was "a result of the actions of one member of staff".
“You gave the council this information on your own child, trusting that we would properly protect it. I believe, therefore, that it is my duty to tell you what has happened and to apologise unreservedly for any distress this matter may cause,” he said in a letter to parents.
At the end of January, Midlothian Council was fined £140,000 by the Information Commissioner's Office (ICO) following three data breaches related to children's social service reports.
Chris McIntosh, CEO of ViaSat UK, said: “Most of us appreciate how easy it is to lose devices such as memory sticks.
“Data protection laws and guidelines are there for a reason and should be followed at all times: the cost and inconvenience of not doing so is easily outweighed by the risk presented to those whose data is left unprotected. In this case, the need to encrypt data, educate staff on the importance of data protection and ensure that unencrypted data cannot be taken off-site is a lesson that hopefully East Lothian Council has now learned.”