Image copyright AFP Image caption South Korean customers had to cancel their credit cards after data was stolen

South Korean regulators have punished three credit card companies for their role in the country's largest-ever theft of financial data last month.

KB Kookmin Bank, Lotte Card and NH Nonghyup Card will each be fined 6 million won (£3,371; $5,640).

They will also be banned from issuing new credit cards for three months.

This comes after financial data on more than 20 million people - more than 40% of South Korea's population - was stolen and sold to marketing firms.

South Korea's Financial Supervisory Commission (FSC) said the three firms had "neglected their legal duties of preventing any leakage of customer information".

Last month, branches of the three firms were swamped with thousands of angry customers looking to cancel their cards or be issued new ones.

The three bosses of the credit card firms involved made a public apology for the breach, and several executives have resigned or offered to step down over the issue.

The data was stolen by a computer contractor working for personal credit ratings firm Korea Credit Bureau, who was arrested last month.

He allegedly stole the data - which included names, social security numbers and phone numbers - by saving it on a USB stick between October 2012 and December last year.

South Korea has one of Asia's highest usage rates for credit cards - an average adult owns multiple cards and will regularly switch companies in order to get better deals or rewards.

However, an increasing number of South Korean firms have been the subject of cyber-attacks.

In 2012, two hackers were arrested for getting hold of the details of 8.7 million subscribers to KT Mobile.

In 2011, details of more than 35 million accounts of South Korean social network Cyworld were exposed in an attack.