The denunciations have begun already. The new BBC Persian television service begins broadcasting today, but Tehran has already described the satellite channel as “suspicious and illegal”. Iranian officials, insisting that it would be “working against the interests of the Islamic Republic”, have spoken darkly of espionage and psychological warfare. Their fears are transparent. With a huge audience already for its Farsi website and radio broadcasts, the BBC is certain to attract an enthusiastic response to its television service. Tehran's monopoly of its airwaves will be broken.

For all authoritarian governments, information is power. The first target of military coups is invariably the television station. Freedom of expression is a prerequisite of democratic freedom, and those who insist on the control of information are those who also want to control a nation's thoughts. But no frontier can seal off ideas. And the BBC, arguably the world's most influential and respected broadcaster, will profoundly influence debate among the 100 million Farsi speakers in Iran, Afghanistan and Tajikistan.

Many Iranians have deep-seated suspicions of alleged British conspiracies, and say that the resignations of the late Shah and his father were in part triggered by BBC broadcasts. For President Ahmadinejad and zealots among the Revolutionary Guards, the BBC represents a mortal danger: ideas can be incendiary among a young, disaffected and largely pro-Western population. Tehran has done its best to stop these spreading. Satellite dishes are banned, although most middle-class homes have them and enforcement is patchy. No reporter for the new television service will be allowed to work in Iran. And periodically the authorities block access to the Persian online news service - regularly accessed by a third of all Iran's internet users.

Tehran's reaction is similar to that of the Soviet authorities who denounced and jammed Western “voices”, especially the BBC. Communism was, in part, defeated because of the commitment of Britain and other countries to broadcasting in Russian. There is a clear political agenda here. The World Service is funded directly by the Foreign and Commonwealth Office, which allocates money according to its political priorities. The aim with the new Persian service is not regime change, but engagement. The World Service, which broadcasts in 32 languages, has long found that truth and independence are more credible than crude propaganda. Like the Arabic television service set up last year, the new station will augment news with programmes on arts, culture, science, technology and sport. A growing number of commercial broadcasters and online operators will also ensure that the BBC will have to compete in the Persian markets.

The Persian service, for which an extra £15 million has been found, has been launched at an opportune moment. The incoming Obama administration has said that it wants to begin a dialogue with Iran. Fresh confrontations still loom, however, over Iran's nuclear ambitions, its support for terrorist groups and opposition to any Middle East peace initiative. Iran is a vibrant, argumentative and divided society that faces elections in May. The battle between pragmatists and hardliners will be intense. The BBC can and will influence that debate.