The European Union has revealed a draft law to give it the power to move the lucrative euro clearing business out of London and keep it in the EU after Britain leaves the Union in 2019.
London currently processes three-quarters of the trade in this financial sector, providing thousands of jobs.
But European Commission vice-president Valdis Dombrovskis said Brexit needed "certain adjustments to our rules".
The law will decide if London will have the right to host the work post-Brexit.
London is currently the world leader for the clearing of all types of currency-denominated derivatives including the euro.
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Clearing is the process by which a third party organisation acts as the middleman for both buyer and seller of financial contracts tied to the underlying value of a share, index, currency or bond.
Trillions of euros are handled through clearing houses every year, mostly through London.
In a statement, Mr Dombrovskis said: "As we face the departure of the largest EU financial centre, we need to make certain adjustments to our rules to ensure that our efforts remain on track,"
The financial industry has warned that forced "relocation" of the work would split markets, increase trading costs, weaken the euro and threaten the jobs associated with the clearing houses in London.