The BBC has said shortwave broadcasts in English of World Service radio are being jammed in China.
In a statement, the corporation said it was not possible to determine exactly where the blocking was coming from.
But it said the "extensive and co-ordinated efforts are indicative of a well-resourced country such as China".
"The BBC strongly condemns this action which is designed to disrupt audiences' free access to news and information," said the statement.
The BBC has experienced several instances of satellite services being jammed in recent years. Shortwave jamming is less frequent, it says, although it has hit transmissions by BBC Persian in Iran and was historically used to block broadcasts during the Cold War.
BBC director of global news Peter Horrocks said the jamming in China was being timed to cause maximum disruption to BBC World Service English broadcasts there.
"The deliberate and co-ordinated efforts by authorities in countries such as China and Iran illustrate the significance and importance of the role the BBC undertakes to provide impartial and accurate information to audiences around the world," he said.