No financial data was accessed and its systems have not been hacked, it added. It said this was a precautionary measure and apologised for the inconvenience.
The supermarket said it had emailed everybody potentially affected (600,000 Clubcard holders), that nobody would lose their points and new vouchers would also be issued. The UK loyalty scheme offers one point for every pound spent in store. Every 100 points are worth GBP 1.
The BBC understands about 19 million people have a Clubcard account.
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