Claimants were sent their annual tax credit award notice, along with personal details such as salary information and the date of birth of other claimants.
HMRC has blamed a ‘print run error’ and says it will be apologising to affected customers and ensuring the mistake is not repeated.
It insists the data included can not result in identity theft.
A HMRC spokesman says: “HMRC takes data security extremely seriously. Unfortunately an error has occurred in one of the tax credits print runs causing some customer information to be wrongly formatted. Investigations are underway to identify the cause of the problem and we will be contacting affected customers in writing this week, apologising and providing a corrected award notice. An initial analysis shows that ID theft could not result from this printing error.”