Moving euro clearing away from London after Brexit could cost investors up to €100bn, the chief executive of the London Stock Exchange Group said.
The comments from Xavier Rolet came after the European Commission launched a consultation, which is expected to be completed at the end of month, over the future of the clearing of euro-denominated derivatives.
Some of the proposals put forward by the commission include insisting euro-denominated transactions can only be cleared within Eurozone member states.
However, writing in The Times on Monday (22 May), Rolet warned that would increase costs and risks.
"This would achieve the opposite of the G20 aims," he said.
"It would increase, not reduce, levels of systemic risk and increase costs for European companies, diverting capital away from the European economy."
Currently, London clears 18 major currencies, resulting in "multi-currency netting efficiencies", which means [London-based European clearing house] LCH saved their clients $21bn in costs last year.
"Strip out euro clearing and you lose these efficiencies, potentially increasing cumulative trading costs by €100bn over five years," Rolet added.
"If business left it would go somewhere with the scale to offer these efficiencies like New York, not Paris or Frankfurt.
"Despite this, US regulators allow the majority of the world's US dollar swaps to clear here, due to sufficiently strong direct oversight of LCH's activities."
In November last year, a report from independent think tank EY warned up to 83,000 jobs could be lost over the next seven years, should euro-denominate clearing leave London.