Mr Brown told reporters at his monthly press conference on February 18, soon after the nationalisation of the troubled lender, that a profit could be made from the business when financial markets recovered.
The statement was made days after his own financial advisers had effectively ruled this out.
Goldman Sachs, which was advising the Treasury on its options for Northern Rock, had already told ministers that a loss of £450m was the best-case scenario and that a loss of £1.28bn was the most realistic base case scenario, according to documents published by the Sunday Times.
The US investment bank also warned that the figure could be much higher if there was a significant drop in house prices, which now looks increasingly likely.
The Treasury said on Sunday its purpose had always been to maintain the stability of the financial sector and making the biggest profit was not its intention.
