International Development Secretary Andrew Mitchell has admitted that his department betrayed the name of an anti-corruption whistleblower in 2009.
The man's identity was passed on to a private equity firm he had accused of investing in corrupt companies.
Mr Mitchell told Newsnight it had been an "inadvertent" error and has issued an unconditional apology to the man.
Dotun Oloko, of Brighton, Sussex, was targeted by private investigators after his name emerged.
Peter Marshall reports.
Broadcast on Thursday 19 January 2012