The BBC's Farsi-language service, BBC Persian, came under a sophisticated cyber-attack by Iran in early March, the broadcaster's director general Mark Thompson said.
Two separate incidents on 2 March -- one an attempt to jam BBC satellite feeds into Iran and another a series of automated calls to the broadcaster's London office in attempt to flood phone lines there -- came shortly after the BBC announced a spike in viewership of its Persian TV service.
The number of viewers of Persian TV has nearly doubled since 2009, rising from 3.1m to 6m the end of last year.
Thompson will address the Royal Television Society this evening. The BBC released excerpts of Thompson's speech, "The BBC in 2012 and beyond," during which he will say:
It now looks as if those who seek to disrupt or block BBC Persian may be widening their tactics.
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.
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.
Thompson has accused Iran of intimidation in the past.
BBC Persian provides Farsi-language television, radio and internet services. The TV portion of the service was launched in January 2009.