More than a year before JP Morgan Chase racked up billions of dollars in losses from bad trades in its London investment office, Bank of England officials raised concerns internally about potential risks arising from some of the office's activities, but didn't formally alert other regulators, according to people involved in the central bank's talks.

In late 2010, employees at the central bank worried that the London arm of JP Morgan's chief investment office had come to dominate some important corners of the city's financial markets-including residential mortgage-backed securities-and they were concerned about the potential impact that could have on the stability of UK markets, these people said.