Two computer discs holding the personal details of all families in the UK with a child under 16 have gone missing.
The Child Benefit data on them includes name, address, date of birth, National Insurance number and, where relevant, bank details of 25 million people.
Chancellor Alistair Darling said there was no evidence the data had gone to criminals – but urged people to monitor bank accounts “for unusual activity”.
The Conservatives described the incident as a “catastrophic” failure.
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