The Inland Revenue has shut down its tax credits website after a security scare.
The UK tax authority said a criminal investigation is underway after it identified and stopped several fraud attempts during compliance checks. It is also introducing new checks.
The fraud concerned internal information on workers at the Department for Work and Pensions (DWP), a spokesperson said, with several DWP identities being used in subsequent fraudulent tax credit claims.
A hotline has been set up for those who believe they may have been affected. Since a criminal investigation into the case is ongoing, further details could not be released, the spokesman said.
DWP provides state financial support to 11 million pensioners and helps people plan for retirement.