The Serious Fraud Office says it has accidentally sent thousands of pages of evidence relating to an investigation into defence giant BAE Systems to the wrong person.
A statement from the office said it identified the data loss in May and that 98 percent of the material has already been recovered.
The office said that none of the material related to matters of national security.
More top news
David Cameron is facing growing pressure for the UK to take in more of the many thousands of migrants fleeing to Europe.
China has put on its biggest display of military might in a parade to commemorate victory over Japan in World War Two.
Hungarian Prime Minister Viktor Orban is to meet with European Union leaders in Brussels today to discuss the migration crisis.