Mrs Long voted against the sale and has previously questioned whether the sale would provide value for money.
Naomi Long MP said: “This report raises serious questions over the decision of the Government to sell Royal Mail. It shows that there was not a proper consideration of the process and as a result the Treasury may have lost out on £1billion.
“I have previously expressed concerns over the impact of privatisation on postal services and the disproportionate impact it would have on elderly people. I queried the Government over what protections would be in place for rural services and how they would ensure that there weren’t any dramatic price rises.
“The Government got it wrong over the sale of Royal Mail. I hope that lessons can be learned from this report and that there is a better decision making process in place for future large scale Government projects.”