Iran has restricted access to Google's email service, Gmail, and search engine.
A firewall already prevents Iranians from accessing many Western sites.
The latest move coincides with protests throughout the Muslim world - including some in Tehran - against an anti-Islamic film posted on Google's video-sharing site YouTube.
A government deputy minister announced the upcoming ban on Sunday on state television.
"Google and Gmail will be filtered nationwide, and will remain filtered until further notice," said an adviser to Iran's public prosecutor's office Abdul Samad Khoramabadi.
The announcement was also sent out as a text message on mobile phones.
The unsecured version of the search engine, which is much easier to eavesdrop on, remains accessible.
The BBC Persian service says: "Google search website is accessible, but is not functioning properly. Google services which need a secure SSL [Secure Sockets Layer] connection are out of reach in Iran.
"Any attempt to get access to those services leads the user to a never-ending waiting phase, where nothing comes up."
Users can only access Gmail accounts by using virtual private networks (VPNs), which allow web surfing behind heavily encrypted firewalls.
Many Iranians already use VPNs to bypass the government's restrictions on other blocked Western websites, said Mahmood Tajali Mehr, an Iranian telecommunications consultant living in Germany.
"This is just a move by the Iranian governement towards a so-called nationwide intranet, to control all the traffic from the outside, and authorities are saying they will implement it in about three years.
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