The BBC has announced that its Persian TV service has almost doubled its audience in two years, despite what it called "persistent and repeated blocking" by the Iranian authorities.

New figures, released today to coincide with the 80th anniversary of the BBC World Service, show the channel's audience has grown from 3.1 million in 2009 to six million last year.

BBC director of global news Peter Horrocks said: "These figures are a tremendous tribute to the courage and dedication of BBC Persian journalists in the face of appalling bullying and intimidation by the Iranian authorities."

The BBC has repeatedly complained of "jamming" - deliberate interference with the Persian channel's broadcast signals - since the service launched three years ago. Last December, five international broadcasters issued a joint statement calling on regulators to take action against those responsible.

A series of special broadcasts are planned on the BBC World Service today to celebrate 80 years on-air.

The event also marks the end of an era for the broadcaster, which is vacating its London headquarters in Bush House to move to the newly refurbished Broadcasting House, where it will share facilities with other parts of the BBC's newsgathering operation.

Horrocks said in a release: "The 80th birthday and departure from Bush House means these are historic and changing times for the BBC World Service. We want our audiences to be at the heart of both the commemoration of the past and conversation about the future."