Iran appears to have blocked the official website for the London 2012 Olympic Games.
Users in Iran have tweeted that they are unable to connect to london2012.com and are instead redirected to peyvandha.ir - a site offering stories from Iran's official news agencies.
Blockediniran.com intermittently suggests Iran-based users are unlikely to be able to see the Olympics pages.
Iran's ministry of foreign affairs did not reply to a request for comment.
Nima Akbarpour, the presenter of the BBC's Click Farsi programme, said such website bans are not uncommon, but it is hard to know exactly who is responsible.
"The blocking process in Iran is not related to a single specific organisation," he said.
"It happens every day - even affecting pro-government sites and blogs. The Iranian government's Internet Filtering Committee is in charge of the process, but individual judges can also order a web filter to be imposed."
Iran's Supreme Leader, Ayatollah Ali Khamenei, recently ordered officials to set up a new body to co-ordinate decisions regarding the net.
Citizens have also been told they would need to show IDs and give their full name when visiting an internet cafe.
President Mahmoud Ahmadinejad has also discussed plans to create a "clean web" within Iran with its own search engine and messaging service.
Iran had previously signalled it might boycott the Olympics over claims that the official logo spells the word "Zion" - a Hebrew word used to refer to Israel or Jerusalem.
In February 2011 the Iranian authorities called for the logo to be withdrawn and the designers "confronted".
However, a follow-up letter later made clear its athletes would still "participate and play gloriously".
The Iranian weightlifting superheavyweight, Behdad Salimikordasiab, is expected to be among those taking part.
He previously won gold in the Asian Games in 2010 despite being affected by swine flu.