The big four banks could be facing class action from thousands of customers who were sold worthless credit cards insurance they didn't need.

At least 100,000 people claim they have paid upwards of $40 million for credit card insurance they do not even qualify for.

Law firm Slater and Gordon is looking at filing proceedings against Commonwealth Bank, Westpac, ANZ and NAB over the insurance which is taken out by customers to cover repayments on cards if they become involuntarily unemployed, sick or injured.

'Worthless' credit card insurance has reportedly been sold to more than 100,000 people in Australia. (AAP) (AP/AAP)

Slater and Gordon claim the insurance, called CCI, was offered to customers who were either not employed, already had their income protected, ruled ineligible because they work for themselves or were over the maximum age.

It comes after Australian Securities and Investments (ASIC) forced the Commonwealth Bank to refund $10 million in CCI premiums paid by 65,000 customers, who were all either unemployed or students.

The CBA last week said it would stop selling CCI ahead of the Financial Services Royal Commission which starts with hearings tomorrow.

Commonwealth Bank has already ditched credit card insurance ahead of the start of the Royal Commission into banking. (AAP) (AP/AAP)

Slater and Gordon senior associate Andrew Paull told TODAY this morning the class action could reach up to $50 million.

"Essentially, this is a form of insurance which is suppose to protect people if they become unwell or unable to make their credit card repayments," Mr Paull said.

Slater and Gordon are taking on the class action on behalf of the 100,000 customers. (AAP) (AP/AAP)

"But, there are certain groups of consumers who are ineligible to claim such as people who are unemployed, retirees, people who are causally or self-employed.

"We know the scripts that used to sell the product don't often mention the exclusions. We encourage people to check their statements or call their bank and ask if they've been paying for the insurance.