Mood's said the political and security situation was more stable and saw signs of economic recovery, but kept the rating at Caa1 saying government finances are still too weak.
The bank said it sold $37.6 million at a cut-off price of 7.1401 pounds per dollar on Monday, unchanged from the last sale on Thursday.
The rates at which banks are allowed to trade dollars are determined by the results of central bank sales, giving the bank effective control over official exchange rates.
In the unofficial market, the pound was trading at 7.47 to the dollar, weaker than Thursday's rate of 7.45 to the dollar, one trader said. Another trader said the pound was trading at 7.475 midday on Monday. The pound has been steadily weakening in the unofficial market for just over a week.
The rate for dollar sales has stayed in a tight range since the beginning of June, after the bank allowed the pound to weaken in May. The central bank offered $40 million on Monday.