LONDON (Reuters) - Pfizer ( PFE ) and BioNTech asked a London court to revoke rival Moderna's ( MRNA ) patents over technology key to the development of vaccines for COVID-19, as the latest leg of a global legal battle began on Tuesday.

Pfizer ( PFE ) and its German partner BioNTech sued Moderna ( MRNA ) at London's High Court in September 2022, seeking to revoke patents held by Moderna ( MRNA ), which hit back days later alleging its patents had been infringed.

The competing lawsuits over the companies' two vaccines, which helped save millions of lives and made the companies billions of dollars, are just one strand of ongoing litigation around the world which focuses on messenger RNA (mRNA) technology.

Moderna ( MRNA ) says Pfizer ( PFE ) and BioNTech copied mRNA advances it had pioneered and patented well before the COVID-19 pandemic began in late 2019.

U.S.-based Moderna ( MRNA ) is seeking damages for alleged infringement of its patents by Pfizer ( PFE ) and BioNTech's Comirnaty shot on sales since March 2022.

Pfizer ( PFE ) made $11.2 billion in sales from Comirnaty last year, while Moderna ( MRNA ) earned $6.7 billion from its vaccine Spikevax, illustrating the potentially huge sums at stake.

Pfizer ( PFE ) and BioNTech, however, are asking the High Court to revoke Moderna's ( MRNA ) patents, arguing that Moderna's ( MRNA ) developments of mRNA technology were obvious improvements on previous work.

The London lawsuits have been split into three separate trials, with one due to consider Moderna's ( MRNA ) 2020 pledge not to enforce its vaccine-related patents during the pandemic starting next.

Pfizer ( PFE ), BioNTech and Moderna ( MRNA ) are also involved in parallel proceedings in Germany, the Netherlands, Belgium and the United States, much of which has been put on hold, as well as at the European Patent Office.