E.C.B. Seeks Control Over London Trading After ‘Brexit’br By JACK EWINGJUNE 23, 2017br FRANKFURT — The European Central Bank took steps Friday to ensure it can oversee trillions of euros in London trading of derivativesbr and other risky securities even after Britain leaves the European Union.br The central bank’s action was an example of how Britain’s vote to leave the bloc — which took place a year ago Friday — could force a realignment of financial marketsbr and undercut London’s status as a global trading center.br The central bank needs “to have enough control in time of crisis.”br London clearinghouses, which dominate trading in derivativesbr and other securities denominated in euros, have become even more powerful in recent years.br Under new rules created to prevent a repeat of the 2008 financial crisis, regulators around the world have pushed banksbr to carry out the trading of derivatives using clearinghouses, where the transactions can be better monitored.br Previously, banks typically traded many derivatives in secret with one another, making it difficultbr for outsiders to know what kind of risks might be lurking in the financial system.