Federal Reserve officials publicly declared it was business as usual in the face of Standard and Poor’s downgrade of US government debt, but privately they acknowledged these were unchartered waters.

Within 90 minutes of S&P’s decision, a joint release from US banking regulators declared that, despite the downgrade of US paper, there would be no change in the risk-weighting of treasury bills, bonds and notes or any paper guaranteed by the US government. In other words, banks do not have to post any additional capital against their Treasury positions.

Regulators also announced that the treatment of US treasuries at the Fed’s discount window would be unchanged. Typically, the riskier an asset, the more collateral banks have to post to borrow from the Fed’s emergency lending facility.