The pound fell in the wake of Boris Johnson's resignation from the Cabinet on Monday.
At 15.00 sterling was trading at $1.3261, down 0.64 per cent from the $1.3346 level at 14.40 when rumours of the Foreign Secretary's departure began circulating.
Against the euro the pound was at €1.1287, down 0.43 per cent over the same period.
The move suggests the departure of Mr Johnson makes a no deal Brexit more likely.
Sterling had taken the resignation of the Brexit Secretary David Davis largely in its stride.