London. The Bank of England is warning that the safety of the financial system could be at risk from big investment banks extending loans against borrowers’ share holdings.

Banks in London lost more than 1 billion euros ($1.1 billion) on a single so-called margin loan deal in 2017, Sam Woods, chief executive of the BOE’s Prudential Regulation Authority, said in a speech in London.

The loans are “not well captured by today’s capital framework for banks,” Woods said. “Complexity creates a weakness in the system.”

“The lesson of financial history is that unless we are absolutely vigilant, and keep questioning both firms and ourselves about evolving and emerging developments in the markets we oversee, then this safety can easily slip through our hands,” Woods said.

The warning was included in a list of threats the PRA is watching closely, including the shift toward higher loan-to-value mortgages and leverage that isn’t being fully captured by post-crisis regulations.