The Yorkshire Building Society (YBS) has said that it will be taking steps to improve its security after a laptop containing unencrypted customer details was stolen.
The laptop was recovered within 48 hours, news agency the Press Association has reported.
The laptop was stolen from YBS offices in Cheltenham.
A forensic test has revealed that no sensitive customer data had been compromised by the thief, although there were several tell-tale signs of attempts made to access it.
The UK Information Commission’s Office (ICO) has discovered that the theft happened after a manager at Chelsea Building Society left the laptop alone on a desk.
"It is extremely concerning that an unencrypted laptop containing large amounts of personal data was left unsecured overnight, together with its passwords. Employees should only have access to information that is absolutely vital to work being carried out,” said Mick Gorill, enforcement head at the ICO.
The ICO has said that it has found YBS to be in breach of the Data Protection Act because of the incident, but has found that the building society has taken "prompt and effective action" to prevent this from happening again.