Mastercard would gain up to £180m a year if the claims of members of a mammoth group action against the card giant are not allowed to be continued by their estates, the Competition Appeal Tribunal has heard.

Lawyers for former financial services ombudsman Walter Merricks – who is bringing a £14bn claim on behalf of around 46 million people – applied to amend his claim form at a hearing on Friday to add representatives of the estates of class members ‘who were alive when the claim form was filed on 6 September 2016, but have subsequently died or will die during the course of the proceedings’.

They argued the ‘domicile date’ for the claim should be 6 September 2016, which ‘prevents the unattractive and unfair position that, going forward, as elderly and ill class members die, the defendants would gain a corresponding benefit from the reduction in their liability’.

Merricks’ barrister Paul Harris QC said in written submissions that this would remove the ‘perverse incentive further to delay the resolution of these proceedings’.

‘To illustrate the benefit that Mastercard would get from the tribunal not allowing representatives of deceased estates to participate in the claim, based on the draft amended pleaded claim, Mastercard would gain roughly at least £78 million (excluding interest) or £180 million (including interest) for each year that the proceedings are ongoing,’ Harris said.

‘People who were alive at the date of the claim form had a perfectly good cause of action’ and they ‘should be allowed to pursue it’, he added.

However, Mastercard contended that the domicile date should be 18 August 2021, the date when the tribunal decided to grant a collective proceedings order (CPO).

Mark Hoskins QC, for Mastercard, said in written submissions that the original claim form ‘expressly excluded claims on behalf of individuals who had passed away before the domicile date’. The tribunal ‘therefore does not have power to permit an amendment that seeks to add or substitute as parties personal representatives of persons alive when the claim form was issued but who died prior to the domicile date’, he added.

Hoskins also argued that Merricks’ proposed domicile date was an ‘attempt to avoid the consequences of the drafting of his original claim form’.

Mr Justice Roth said the CAT would deliver its ruling on the domicile date in the near future.