The BBC’s director - general will reveal how the BBC is subject to persistent intimidation by the Iranian authorities, and suffered a “sophisticated cyber attack.”

BBC headquarters were targeted in a cyber attack on March 2, disrupting email access within some parts of the BBC for about four hours. The website is also believed to have been targeted by a DDOS attack, which overloads websites with repeated automatic requests for information. No broadcasting was affected.

Director - general Mark Thompson, who is due to give a speech at the Royal Television Society today, said members of staff were constantly intimidated into leaving the BBC Persian service, and pressured into becoming Iranian police informers.

The BBC released an extract from the speech, describing the attempted cyber attack. Mr Thompson will tell the RTS “"It is difficult, and may prove impossible, to confirm the source of these attacks, though attempted jamming of BBC services into Iran is nothing new and we regard the coincidence of these different attacks as self-evidently suspicious."

"It now looks as if those who seek to disrupt or block BBC Persian may be widening their tactics."

He could not say definitely if the attacks were the work of the Iranian government. Mr Thompson has previously spoken of Tehran’s attempts to jam the BBC’s broadcasts to Iranians. The BBC recently announced a huge increase in listeners to its Persian broadcasts, doubling in the past two years to around six million.

Mr Thompson told the Andrew Marr show in February that the sister of a BBC staff member working for the Persian service was detained, intimated and held in solitary confinement by the Iranian authorities.