Asset managers have written to the European Central Bank saying that some bank bonds could ultimately become “uninvestable” unless rules are clarified governing when investors would be forced to take losses.

A working group set up by the International Capital Market Association's Asset Management and Investors Council - which represents the likes of BlackRock, Pimco, Citadel Securities, Allianz Global Investors, and BNP Paribas Asset Management - wants the ECB to help standardise when investors are "bailed in" in the event of a bank failure or default.