Iran will attempt to block Google, Gmail and YouTube in response to an anti-Islamic film which has sparked deadly protests across the world.
A Iranian deputy government minister said on Sunday that Google services would be blocked "within a few hours".
"Google and Gmail will be filtered throughout the country until further notice," said the official, identified as Abdolsamad Khoramabadi.
But according to several reports, Google services were still accessible in some areas.
The Guardian reported that citizens in Tehran were reporting problems with Gmail, but in the city of Isfahan the service was still accessible.
Iran has previously blocked Google services temporarily. In February many users in Tehran reported outages lasting about four days before service was restored.
The announcement was made via state television, and comes alongside promises to switch its citizens onto a 'domestic internet' in what many fear is a renewed attempt to curb access to free media - but which Iran said was intended to boost online security.
Several powerful attacks have been launched against Iranian computer networks in recent years, including the famous Stuxnet virus and the recent Flame malware, described by researchers as the most powerful ever discovered.
Iran already has restrictive filters on the Internet, which blocks access to dozens of sites on grounds of taste and legality.
Other countries - including the UK - block certain sites from most users, but in Iran the list of restricted domains is much more extensive.
It is relatively simple to overcome these filters using Virtual Private Networks, which make the computer or device appear as if it is based in another part of the world. But experts suggest that if Iran was able to build its own, internal internet, it might be possible to more rigorously restrict access to the wider web.
Officials said recently that all government agencies are connected to the so-called National Information Network, and that citizens would be switched by March next year.
But the detail of the plans is unclear, and it is not known whether access to the global web would be cut once the new network is established.