Letters sent out to patients about the national NHS records database contained confidential data about other people, the BBC has learned.
A review has taken place after letters containing personal information were sent to the wrong patients in Cambridgeshire, BBC Look East found.
The letters contain details about which surgery the person attends and their NHS number.
NHS Connecting for Health is writing to patients affected to apologise.
NHS Connecting for Health (CoH) said: “As part of our plans to inform patients about the Summary Care Record programme, NHS Primary Care Trusts (PCTs) have written personally to everyone in England over the age of 15 and a half.
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The vendor involved in the mailing was not named in the story.