The Home Office has admitted breaching data protection rules when it launched the Windrush compensation scheme.
The department sent information to Windrush migrants in a way that meant their email addresses could be seen by other people.
An internal review has been launched and the matter has been referred to the Information Commissioner.
Immigration Minister Caroline Nokes apologised “unreservedly” for what she said was an “administrative error”.
The data breach affected five batches of emails, each with 100 recipients, Ms Nokes added.
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The recipients included Windrush migrants and other people who had asked to be kept informed about the compensation scheme, which was launched last week.
The Home Office revealed the error in a written statement, which also clarified other details about the scheme.