A computer glitch has left 3.3 million UK Post Office card account holders unable to complete transactions at branches.
The problem also means credit and debit card holders cannot make payments using PIN pads at the Post Office's 11,820 branches.
Special arrangements have been made to ensure that emergency cash payments get made to pensioners and benefit claimants using the Post Office account. ATMs are unaffected.
Martin Moran, commercial director, Post Office, says: "We apologise to customers for any inconvenience. We are making every effort to restore these services as soon as possible."