When asked if Sir Keir was the right man to take the country forward, Brown said “this is obviously serious”.
But he added: “I can look in his eyes and I can see that he is a man of integrity. He wants to do the right things.
“Perhaps he’s been too slow to do the right things, but he must do the right things now, and let’s judge what he does, on what happens in the next few months when he tries to, and I believe (he) will try, to clean up the system.”
Email exchanges released in the latest Epstein files suggest Mandelson, who was then business secretary, gave Epstein almost minute-by-minute updates in the days following the 2010 general election, when Labour lost its majority.
Brown said he believes Peter Mandelson “betrayed” his country by leaking the information.
He said Mandelson’s communications with Epstein “meant Britain was at risk because of that, the currency was at risk, some of the trading that would happen would be speculative as a result of that and there’s no doubt that huge commercial damage could have been done and perhaps was done”.
Brown said he felt “shocked, sad, angry, betrayed, let down” by the messages.
He said they indicate Lord Mandelson was planning for a career outside government while he was business secretary during the response to the global financial crisis.
“The emails show that,” he said. “They show that he was talking about writing a book while he was supposed to be a government minister. He was talking about how he would get a job after the government was over and talking about what banks would employ him.
“All these things were happening at one and the same time, so it was a complete betrayal of his colleagues and of the job that he was carrying out and, of course, it was a betrayal of the country.”