The Metropolitan Police accidentally shared the email addresses of more than 1,000 victims of crime with others.
The mistake occurred when "human error" led to their details being inserted in the wrong box of an email sent by the Met to distribute a quality control survey.
This was sent to 1,136 people in seven batches, meaning the email addresses of those affected were seen by up to 197 other people.
A Scotland Yard spokesman said: "We can confirm that the Metropolitan Police Service (MPS) has inadvertently shared the email addresses of a number of victims of crime with other victims.
"The sharing of the data was the result of human error and occurred on Monday January 30 when the Crime Recording and Investigation Bureau (Crib) project was emailing out a survey to ensure that victims were receiving a better service as part of the MPS creation of a single telephone investigation unit for London.
"In total, 1,136 emails were sent out in seven batches of between 119 and 198 recipients but because the addresses were put in the wrong box they were visible to the other recipients in the batch.
"No other personal details were revealed and we are contacting everyone affected to explain what happened and to apologise."
The Met said it was reviewing how it carries out surveys to avoid similar mistakes being made in the future.
Nick Pickles, director of privacy group Big Brother Watch, said: "The fact that technology makes it easier to make mistakes does not excuse the mistakes.
"Too many incidents where information is compromised are treated as minor, internal problems.
"Given how much personal information is now collected and held, it is time for the sanctions to be far greater and real action taken to protect people's privacy."
The Information Commissioner's Office said: "We have recently been informed of a possible data breach which may involve the Metropolitan Police.
"We will be making inquiries into the circumstances of the alleged breach of the Data Protection Act before deciding what action, if any, needs to be taken."