Some of the largest investment firms in the US have been overcharged by banks for currency trades, bank insiders and others claim, broadening the scope of alleged abuses in pockets of the $4 trillion foreign-exchange market.

BlackRock, the world's largest fund manager, became concerned at the rates it and its clients were charged for some currency trades by custody banks including Bank of New York Mellon, according to an internal BlackRock investigation about a year ago and people familiar with the firm.