Mumbai, Oct 20 (IANS): In pre-Diwali panic among consumers, over three million debit cards of various banks are believed to be 'tainted' following a suspected security breach, even as investigations have begun into the causes behind the security risks.

Several banks, including the State Bank Of India (SBI), have already started blocking their customers' debit cards and re-issuing fresh ones to them free of cost.

An estimated more than 30,00,000 debit cards issued by various public or private banks are said to be affected by a potential risk of data breach.

Debit or credit cards are prone to security issues when unauthorized parties gain access to the confidential data embedded on them, even if it is being swiped in an automatic teller machine (ATM).

In the current scenario, the SBI alone has blocked more than 600,000 debit cards while assuring that the malware-related security breach was reportedly detected in the non-SBI ATM network.

The move has been undertaken to ensure that customer's confidential personal data is not compromised while swiping them for various transactions.

One of the card network companies MasterCard said on Thursday that its "own systems have not been breached".

"We are working on the investigations with the regulators, issuers, acquirers, global and local law enforcement agencies and third party payment networks to assess the current situation," a MasterCard spokesperson said.

It has advised the consumers to review their account statements and activity, and if any unusual or fraudulent transactions are suspected, they should contact the concerned bank for more assistance.