Royal Bank of Scotland saw the biggest one-day drop in its share price for 20 years on Monday, with the stock tumbling by more than a quarter, as Standard & Poor’s, the rating agency, cut the bank’s long and short-term counterparty credit ratings.

The agency cut the ratings from AA-/A-1+ to A+/A-1. It also lowered its long-term counterparty credit ratings on RBS subsidiaries including NatWest and Ulster Bank to AA- from AA.