The supermarket has also started an internal investigation into incidents in which Clubcard vouchers were reportedly stolen from interent-operated accounts.
Tesco has sought police investigation into claims that vouchers of its Clubcard customers have been stolen online.
"We have launched a thorough investigation into a small number of incidents and referred the matter to the police," Tesco said in a statement.
"In the meantime, we'd like to ask any customers who believe they're affected to contact us directly so that we can make sure their accounts are up to date."
Clubcard customers reported losing vouchers collected over a number of years after logging into their Clubcard online accounts, while others were not able to log on to their accounts and some had their account name changed.
The Clubcard scheme is aimed at allowing users to cash in discounts and other items through points gained from shopping at the supermarket.
Tesco has refunded some of the affected customers.