The Serious Fraud Office has admitted accidentally sending a huge cache of confidential documents from an investigation into Britain's biggest arms firm, BAE, to the wrong person.
It did not realise for up to a year that it had misplaced the material which comprised 32,000 pages of documents, 81 audio tapes and computer files.
The material had originally been given to the SFO by 59 sources that helped the agency during one of its most high-profile investigations.
The SFO is not identifying at the moment the individual who inadvertently received the documents, nor did it spell out what they contained.
The documents had been compiled during a six-year investigation into allegations that BAE had paid bribes around the world to secure lucrative arms contracts. At its most controversial point, Tony Blair's government stopped the SFO from investigating alleged bribes to Saudi royals in a £43bn arms deal.
Three years ago, BAE paid out almost £300m in penalties to bring the long-running corruption investigation to a close.
The revelation is the latest blow to the SFO which has experienced the collapse of key cases and embarrassment after paying large sums of money to departing staff without the required approval.
On Thursday, the SFO revealed that documents from the investigation had gone missing between May and October last year. The agency made the discovery in May this year and so far has recovered 98% of the material.
Eddie Cunningham, one of the whistleblowers who gave confidential information to the SFO during its investigation, said: "I am quite astounded. My concern is who got this information and whether it has been copied. This gives one a lot of concern about passing information to a government body that had been so lax."
He received a letter on Thursday from the SFO which admitted that the data could have been copied or shown to others.
An SFO spokeswoman said: "The SFO has a duty to return material to those who supplied it, upon request, after the close of an investigation. In this instance the party requesting the return was sent additional material which had in fact been obtained from other sources." The SFO said the recipient of the mistake was not BAE.
"Any loss of data is a serious matter and the SFO has taken action to ensure no further material can be wrongly sent out," she added.
The agency called in Peter Mason, former director of security at the Palace of Westminster, to conduct an initial review. David Green, the director of the SFO, also launched an independent wide-ranging review of its business processes.
The SFO started its investigation into BAE in 2004 following allegations of corruption uncovered by the Guardian. It examined alleged bribes to a number of countries including the Czech Republic, Romania and South Africa.
In 2010, in a simultaneous deal with the US department of justice and the SFO, BAE pleaded guilty to false accounting and making misleading statements over its contracts with countries including Saudi Arabia, the Czech Republic and Tanzania.
The arms firm was fined £500,000 at Southwark crown court in December that year after admitting it had failed to keep adequate accounting records over the contract for the supply of an air traffic control system to the Tanzanian government.
Labour MP Emily Thornberry, shadow attorney general, said: "This is government incompetence of the first magnitude. The SFO has stumbled from shambles to shambles, with the attorney general completely failing to get a grip."
Thornberry said the account of what has happened raised more questions than answers.
She said: " How did this happen? What category of documents are we talking about here? Who received them by mistake? How much of the evidence has been destroyed? What is the government doing to ensure that this never happens again?
"People will be wondering how many other skeletons there are in the SFO cupboard that the attorney general is aware of but is declining to make public."
The missing data comprised 3% of all the data in the investigation, according to the SFO.
