The UK Serious Fraud Office (SFO) has been branded "incompetent" by a senior politician after admitting that it accidentally sent a huge cache of confidential documents and recordings gathered as part of a corruption investigation to someone not entitled to see them.

In a statement, the SFO said the breach related to 32,000 documents, 81 audio tapes and other electronic media that were accidentally "returned" to an unnamed individual between May and October of 2012.

The material formed 3 percent of the evidence gathered by an enquiry into alleged corruption connected to the hugely-troubled BAE Systems al-Yamaha arms deals with Saudi Arabia dating from the 1980s to 2006.

It is not an exaggeration to describe this as one of the most complex investigations ever undertaken by the SFO, which will serve to amplify the embarrassment over the loss.

"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," explained the SFO.

Ninety-eight percent of the material had been recovered by the SFO, with none of what had been misdirected relating to national security or the 2010 conviction and fine of BAE Systems, the organisation added.

"Any loss of data is a serious matter and the SFO has taken action to ensure no further material can be wrongly sent out." Former Palace of Westminster security director Alan Mason had made some initial recommendations while the SFO has instigated an independent review of it processes by former senior civil servant, Alan Woods, as well as reporting the matter to the Information Commissioner.

All 59 sources affected by the breach have been contacted.

Labour's Shadow Attorney General Emily Thornberry was more caustic in her assessment.

"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," she said in a statement.

"His account of what has happened raises more questions than it answers. How did this happen? What category of documents are we talking about here? Who received them by mistake?, " she said.

"Incompetence like this threatens to have an impact on the reputation of the UK and its relations overseas."