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.

His children were followed to school, he was secretly photographed at his home and church in Brighton and investigated on two continents.

Mr Oloko had complained that funds from the UK's Commonwealth Development Corporation (CDC) were being invested in companies suspected of involvement in money laundering in Nigeria, where he worked.

He took his concerns to the Department for International Development (DFID) in 2009, securing a promise it would not pass on his identity.

But his complaints were handed to the CDC, which turned them over to the private equity firm whose investments Mr Oloko had questioned.

Electronic documents

It in turn sent a private investigator to investigate its accuser.

After two years of denials from his department, Mr Mitchell admitted to Newsnight the department had "inadvertently" passed on original electronic documents, failing to realise Mr Oloko's name was revealed in the electronic properties.

The slip-up happened under the previous Labour administration.

Mr Mitchell offered an unconditional apology and said there would now be a full review of procedures.

He said: "It is vital that whistleblowers should be able to report their concerns with impunity.

"While I am confident DFID acted in good faith to protect his identity, this should not have happened. We apologise unreservedly and are very sorry for any distress caused."

Mr Mitchell said the coalition government had a zero-tolerance approach to corruption and placed the highest priority on efforts to tackle it.

"I have immediately ordered a review of departmental processes to learn the lessons of this case and ensure that whistleblowers who wish to remain anonymous will always be afforded the protection they deserve," he added.

Retribution fear

But Mr Oloko said he was not impressed by what he called the cynical attempt at damage limitation.

He fears his safety is now in jeopardy in Nigeria where attempts have been made on the lives of anti-corruption campaigners.

"At some point or other I will have to pay the price for what I've done," he said.

Asked what he meant, he responded: "There'll be some form of retribution."

A statement from CDC said: "We are sincerely sorry about the harassment the report's author and his family have suffered.

"CDC takes confidentiality very seriously so as to safeguard the identities of people who raise issues with us. It is important that people feel confident that they can bring matters that concern them to our attention."

The statement added that neither the name nor the contact details of the report's author had been apparent to CDC.

"We are taking steps to ensure that this does not happen again and are issuing internal guidance to prevent any inadvertent sharing of embedded author data in future," it added.