The report, to be released this week, could anger rival bidders because it is believed that the NAO has stuck to a neutral line on the sale.
Critics of the way it was conducted include the Co-op Bank and the organisers of a failed management buyout for the business. They believe they were not treated fairly by the Post Office, the Department of Trade and Industry and Schroders, the Post Office's merchant bank.
The main concern of the critics covered the 15 months during which Alliance & Leicester had sole negotiating rights.
The Co-op offered a similar cash sum to Alliance, but executives involved at the time were concerned that no allowance was made for the difference in time value of the offers during a period of high interest rates.
Alliance, as a building society, had to go through time-consuming procedures, but other bidders could have completed much faster if they had been given the chance. Alliance reduced its price just before the deal went through.
The inquiry by the National Audit Office began after Labour MPs criticised the sale price of Girobank as too low.