European Union governments and politicians have agreed new rules that could force large foreign clearing houses with operations in the bloc to relocate there if they want to continue servicing their EU clients.
Euro clearing has been one of the main battlegrounds between London and Brussels in talks that will shape how Europe's financial market is divided up when Britain leaves the EU.
"Today's agreement is essential to achieving legal certainty on the rules that will apply in the future, in particular as regards the way firms based outside the EU will be able to operate in the single market," Eugen Teodorovici, Romania's finance minister said.
The new rules would apply to large US security houses, such as CME and ICE, and British clearing firms after Britain leaves the EU.
The strictest relocation provisions are likely to hit LCH, a unit of the London Stock Exchange, which dominates clearing of euro-denominated derivatives.
That move could strip London of a chunk of the multi-trillion-euro derivative clearing business.
Clearing houses, also known as central counterparties, sit between two sides of a financial trade to ensure it is smooth and completed safely.
Their importance has increased since the financial crisis because regulators have pushed for more derivatives to be cleared by third parties in a bid to reduce risks.
Under the reform, which was proposed by the European Commission in June 2017, foreign systemic clearing houses who offer their services to EU clients would be subject to the bloc's supervision, in addition to the oversight of their national regulators.
Those who clear very large amounts of euro-denominated derivatives, such as LCH, could be even forced to relocate to the bloc to keep their EU clients.
The move is upsetting London and has also been criticised by the US financial regulator, which has threatened retaliatory measures.
As a temporary measure meant to avoid market disruptions in case of no-deal Brexit, the EU markets regulator in February authorised the three UK-based clearing houses LCH, ICE Clear Europe and LME Clear to continue serving EU clients for a year after Britain leaves the EU.