A doctor lost an unknown number of patient details on an unencrypted data stick, according to the Information Commissioner's Office.
A junior doctor working for the East and North Hertfordshire NHS Trust lost the patient details on a train earlier this year, the trust said in a statement on Tuesday.
"In May 2010, a junior doctor acted against Trust procedures and saved brief handover details to an unencrypted USB data stick which they then lost on a train," said the trust. "They acted quickly to report the loss."
The data, which was part of a handover between two doctors at the end a shift, included basic medical information, a trust spokesperson told ZDNet UK on Tuesday.
The Information Commissioner's Office (ICO) found the NHS trust to be in breach of UK data law, it said in a statement on Monday.
The ICO said that the trust did not know how many details were on the stick, as the stick was used on a rolling basis to transfer details between shifts, an ICO spokesman told ZDNet UK on Tuesday.
The trust spokeswoman declined to say whether the trust knew how many patient details had been lost.
"We'd prefer not to answer that question," the spokeswoman told ZDNet UK.
The trust signed an undertaking to follow steps to protect data, including clarifying its policy on the use of portable media.