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The Iranian government has hacked BBC Persian TV as part of broader attempts to disrupt the service, the BBC director-general has confirmed.
Analysts at ZDNet said that the BBC's London office was targeted with automatic phone calls and the company's satellite feeds into the Islamic Republic were blocked.
BBC director-general Mark Thompson was expected to condemn Iran's efforts to "undermine" the service when he gave an address to the Royal Television Society.
Opponents of the BBC's Persian service "may be widening their tactics", he warned.
"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," he said.
"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. We are taking every step we can, as we always do, to ensure that this vital service continues to reach the people who need it."
All BBC Persian service's journalists work outside the country because of Tehran's grip on foreign media. BBC Persian is available via TV, radio, and online services in Farsi. Its audience in Iran almost doubled between 2009 and 2011 to six million, despite the Tehrans campaign of censorship and intimidation against staff.