Hackers launched a "sophisticated cyber-attack" on the BBC, director general Mark Thompson has revealed.
The attack is an attempt to undermine the broadcaster's Persian Service which broadcasts to Iran, Mr Thompson said.
The BBC boss is expected to discuss what he described as the Iranian government's attempt to "undermine" the service during a speech to the Royal Television Society later.
He will say opponents of the service "may be widening their tactics" by engaging in cyber-attacks.
Mr Thompson will say: "There was a day recently when there was a simultaneous attempt to jam two different satellite feeds of BBC Persian into Iran , to disrupt the service's London phone lines by the use of multiple automatic calls, and a sophisticated cyber attack on the BBC.
"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."
The BBC has already said Iranian-based members of staff working for the London-based service have faced intimidation.
Mr Thompson will say: "Arrest without charge, solitary confinement, threats and inducements to persuade their relations either to leave the service or to inform on it secretly to Iranian intelligence: this is how the Iranian government is trying to undermine the BBC Persian Service."
Mr Thompson will go on to say that the BBC "believe that there is no substitute for eye-witness journalism carried out by professionals on the ground" despite "multiple threats" to freedom of speech.