The EU has today adopted a crucial emergency measure to safeguard the stability of financial markets after the Brexit transition period ends on December 31. 

The decision came as negotiations on future relations between EU and UK have descended into acrimony, with the increasing likelihood of a chaotic, "no-deal" scenario. 

The plan is to preserve the stability of the market in derivatives, a complex but vital financial instrument that allows traders to insure themselves against sudden interest or currency exchange rate swings. 

The EU derivatives market is almost exclusively centred on London and was notionally valued in 2018 at €660 trillion - nearly 300 times the size of Britain's entire economy.

In the EU's plan, institutions where these transactions are finalised would have a licence to keep working with EU traders for another 18 months after December 31. 

It will allow European firms until mid-2022 to handle the key service, currently executed by the London Stock Exchange and other clearing houses. 

"Clearing houses play a systemic role in our financial system," Valdis Dombrovskis, an EU Commission executive vice president, said in a statement. 

"We are adopting this decision to protect our financial stability, which is one of our key priorities," he added. 

The next formal round of negotiations on the future relations agreement will take place in Brussels next week. 

Already at an impasse, the talks were further undermined by Britain's wish to go back in part on its divorce agreement, signed in January, to the fury of the Europeans.