Peter Mandelson has dramatically claimed he was rushed into resigning on the basis of incomplete evidence.
He says he agreed to quit his Cabinet job as Northern Ireland Secretary in a moment of personal weakness.
In his first public comments on the Downing Street discussions which led to his resignation, Mr Mandelson paints a vivid portrait of a pressurised meeting in which he was left 'isolated' and unable to defend himself.
He insists he had not lied about his involvement in the Hinduja brothers' passport application, and swore to clear his name through the investigation into the affair, which is being carried out by Sir Anthony Hammond QC.
Writing in the Sunday Times, Mr Mandelson sets out step by step the series of events which led to the collapse of his high-flying political career on Wednesday.
He says: "A small mistake - a failure to focus on a small matter - had turned into a monumental disaster.
"I know I have a mountain to climb before reversing this error, but I know I have to start somewhere."
Mr Mandelson says he has already contacted Sir Anthony and asked for an appointment to give him his version of events, which he feels sure will vindicate him.