Millions of customers face paying charges to deposit cash with Royal Bank of Scotland and NatWest if the Bank of England cuts interest rates below zero.
The state-backed bank has written to 1.3m business clients to say record low rates “could result in us charging interest on credit balances”.
A move to introduce negative interest rates would render RBS the first bank in the UK to make customers pay for depositing cash. The bank said it would not affect personal customers if it came into force.
The plan comes amid speculation of a rate cut and analyst views that rates could remain lower for longer following the vote to leave the EU. UK interest rates have been stuck at a record low of 0.5 per cent since March 2009.
Economists are forecasting that the BoE could reduce rates to 0.25 per cent when it meets next week.
However, Mark Carney, BoE governor, told MPs in April: “We think we could move base rate closer to zero but have not said we have an appetite for negative interest rates.”
Small businesses say they are “deeply concerned” about the warning from RBS.
Mike Cherry, chairman of the Federation of Small Businesses, said: “It is now vital that all finance providers holding deposits from small businesses do everything they can to update customers concerned about any changes to their business current account during this uncertain economic period.”
In its letter to business customers, RBS said a move to charge interest would depend on “future market conditions”. It said “global interest rates remain at very low levels and in some markets are currently negative”.
Ray Boulger of broker John Charcol said: “The day after the referendum the Bank of England contacted banks to ask them what would be impact of negative interest rates. As a result of the responses, it is unlikely the Bank will make them negative.
“If corporate customers have to pay a percentage to have money on a current account, I’m sure a lot of would look to manage the way they handle their banking differently — one solution would be to move banks or switch accounts.”
RBS, which owns NatWest, said: “We will consider any necessary action in the event of the Bank of England base rate falling below zero but will do our utmost to protect our customers from any impacts. We have no current plans to pass negative rates through to personal or business customers.”
Negative interest rates have been employed by banks in Europe, with the European Central Bank and Swiss National Bank charging lenders to deposit cash.
RBS is set to report its second-quarter earnings results in early August. Analysts forecast the bank will report a net loss of £247m.
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