Brevan Howard, one of the world’s biggest hedge funds betting on bonds and currencies, suffered a loss of approximately $670 million as the European Central Bank shocked markets with lower-than-expected stimulus this month.

Brevan's flagship $21 billion macro fund, which until last year had never lost money in a calendar year, fell 3.2% in the first four days of this month, according to a person who has seen the performance numbers. Such a loss is relatively rare for the fund.