LONDON (Reuters) - Wall Street bank JP Morgan and Greek fintech boss Haris Karonis both claimed success on Thursday in a London court battle over how to value a joint business, paving the way for its potential sale.

Karonis's WEREALIZE.COM (WRL), the majority owner of Athens-based Viva Wallet, has accused JP Morgan of trying to hamper the growth of the payments platform and both sides sued each other in London.

But a London judge dismissed suggestions that JP Morgan had any incentive to depress Viva's value and set out how the business should be valued, a judgment published on Thursday showed.

The U.S. bank bought a 48.5% stake in Viva Wallet, which is used by businesses in southern Europe, for about $800 million in 2022. WRL owns 51.49%.

Under the terms of the deal, WRL loses its right to reject any JP Morgan offer to take control of Viva if the business is valued at below 5 billion euros ($5.4 billion) by July 30, 2025.

"As the founder of this business, I am thrilled that Viva will now be properly valued on the basis of its growth strategy in the U.S., reflecting its fair market value," Karonis said in a statement.

JP Morgan said it remained committed to developing and expanding Viva in partnership with WRL, adding that the judgment was a "great outcome".

"With a financial stake in the company, we have repeatedly offered ways to help the company expand and succeed. The court has now provided a critical step to move forward with fair and transparent valuations - which could allow Viva to be sold soon, before the Fintech M&A market further softens," a JP Morgan spokesperson said.

($1 = 0.9315 euros)

(Reporting by Kirstin Ridley. Editing by Jane Merriman)