The Bank of England (BOE) told banks to start getting ready for negative interest rates, while also saying that message shouldn't be taken as a signal that the policy is imminent.

The central bank's Prudential Regulation Authority (PRA) said most financial institutions aren't sufficiently prepared for the prospect of negative rates, especially as regards to retail products like rate-tracking mortgages, so they should take at least six months to set their systems up without running undue risks. The BOE made the announcement as it held its key rate at a record low of 0.1 percent.

The move raises the threat that the central bank will push borrowing costs below zero, turning the lending business on its head by charging depositors to store cash while giving money back to borrowers. The BOE has been studying the case for negative rates for almost a year as a way to pull the U.K. out of its worst slump in three centuries. So far, it hasn't followed counterparts in Europe and Japan in implementing the policy.