In July, a Competition Appeal Tribunal (CAT) judge blocked the interchange fees suit, deciding that the claims were not suitable under the current collective actions regime. The claim was led by former financial ombudsman Merricks and brought under the Consumer Rights Act of 2015 which paved the way for class action lawsuits in the UK.

But it was dismissed for two main reasons. The CAT decided that there was not enough evidence to show the extent to which all businesses had passed on the "illegal fees" to consumers in higher prices. In addition, the CAT wanted Merricks to establish the loss suffered by every one of the 46 million people in the class.

Merricks says that the tribunal was wrong in its analysis and his lawyers, Quinn Emanuel Urquhart & Sullivan, have filed an application for permission to appeal its decision. Mastercard has until 8 September to respond in kind. What happens then is uncertain; Merricks may have a direct right of appeal to the Court of Appeal or may need to got to the Administrative Court for a judicial review.

Says Merricks: "I believe that the Tribunal was wrong in its analysis and in the legal test that it applied. The conclusion that it would not be enough for me to prove the loss suffered by the class as a whole and that I needed to show that I could calculate the actual loss suffered by each individual consumer cannot be correct.

"The government decided that a new regime was needed to allow consumers to recover the losses caused to them by illegal, anticompetitive conduct engaged in by big business. If I can establish the total amount of harm that Mastercard has caused to UK consumers, then why should consumers then get nothing at all if I cannot calculate the precise loss that each individual consumer suffered?"