LONDON (Reuters) -Apple must pay a U.S. patent holder $502 million for the use of 4G patents in devices including iPhones and iPads, London's Court of Appeal ruled on Thursday, in the latest stage of a long-running legal battle.

Texas-based Optis Cellular Technology LLC sued Apple in London in 2019 over its use of patents which Optis says are essential to certain technological standards, such as 4G.

London's High Court ruled in 2023 that Apple should pay Optis a total of $56.43 million plus interest to cover past and future sales over a set period.

But Optis argued that was far too low and challenged the decision at an appeal heard in February and March.

The Court of Appeal said in a partially-redacted written ruling that Apple should pay a lump sum of $502 million, not including interest, for the period from 2013 to 2027.

The lump sum relates to a global licence to use Optis' patents.

An Apple spokesperson said they were "disappointed by this decision and plan to appeal".

"Optis makes no products and their sole business is to sue companies using patents they buy," the spokesperson added. "We will continue to defend against their attempts to extract unreasonable payments."

Optis did not immediately respond to a request for comment outside U.S. office hours.

Thursday's ruling is the latest decision in the legal battle between Apple and Optis over the FRAND (fair, reasonable and non-discriminatory) terms to use Optis' patents.