Credit card giant MasterCard could be forced to pay back £19bn to shoppers who say they paid illegally high transaction charges for 16 years.

The claims – which are likely to average hundreds of pounds per customer – concern the fees levied each time shoppers use their cards.

Former chief ombudsman of the Financial Ombudsman Service Walter Merricks is leading the fight, saying consumers paid more than they should have for millions of purchases from 1992 to 2008.

MasterCard strongly denies the claim, insisting that credit card payments offer value for money to consumers.

The claim is being made under the recently introduced Consumer Rights Act 2015, which enables people to group together to bring US-style “class actions” for the first time in the UK.

Consumer organisation Which? is supporting the claims, saying it is only right that people should be refunded money they have been unlawfully charged.

Campaigners say the fees breach European Union antitrust laws. The EU Council and European Parliament introduced caps of 0.2% for debit cards and 0.3% for credit cards last year.

Earlier this month MasterCard was ordered to pay £68.58m plus interest to Sainsbury’s after losing a related case over debit and credit card fees at the Competition Appeal Tribunal.

The tribunal upheld the supermarket group’s argument that the fees restricted competition.

Other retailers including Asda, Marks & Spencer and John Lewis are lining up similar claims against MasterCard and rival Visa that could total more than £1.2bn.

“Cases such as this illustrate the role European Union laws can play in helping to protect the average person from being ripped off by giant multinational corporations,” said James Pritchard of Macks Solicitors.

“It’s not clear what will happen in these areas which are reliant on European law after we leave the EU. There are many similar regulations that many people might not know exist. If suddenly none of them apply it would make things very interesting. That’s an issue for the lawmakers though!”