Google said on Wednesday it had seen a “sharp drop” in traffic to its Gmail service from Iran, apparently as a result of censorship, amid signs of a stringent crackdown on the US internet group to coincide with the anniversary of the Iranian revolution.

Iranian officials have also said they will impose a permanent block on Gmail. This was a significant escalation of their normally sporadic attacks on the service, said Ronald Deibert, an associate professor at the University of Toronto and head of Citizen Lab, which monitors global internet censorship.

While the threat of a permanent ban could not be confirmed late on Wednesday, the US state department was quick to issue its own condemnation. “The Iranian government seems determined to deny its citizens access to information [and] the ability to express themselves freely, network and share ideas,” a department official said. He added that the Iranian people would “find a way to overcome the obstacles the Iranian government put in their way”.