Britain accused Iran on Sunday of blocking a website days after it was launched by the British government to reach out to Iranians, in the latest spat over media censorship.
British Foreign Secretary William Hague said the "UK for Iranians" website which he launched on Wednesday with a personal video message had been blocked three days later by Tehran.
"I condemn this action by the Iranian government. We have no quarrel with the Iranian people and regret that the Iranian authorities fear their own citizens' interaction and involvement with the outside world," Hague said in a statement.
Britain is at the forefront of Western opposition to Iran's nuclear program, supporting tough sanctions against Tehran which it fears is seeking the bomb. Tehran says its atomic activities are entirely peaceful.
Relations hit new lows last November when protesters stormed the British embassy, prompting London to evacuate its Tehran staff and expel all Iranian diplomats from Britain.
Iran said that was an over-reaction and accused London of censorship when its state-run English language news channel Press TV was banned from British airwaves by media regulator Ofcom in January.
Britain's public service broadcaster, the BBC, said on Wednesday it had suffered a sophisticated cyber-attack following a campaign by Iranian authorities against its Persian service.
Hague said the website (http://ukforiranians.fco.gov.uk/en in English and http://ukforiranians.fco.gov.uk/fa in Farsi) and the use of social media such as Facebook and Twitter were an attempt to engage with Iranians and explain British policy.
Iran blocked a similar website set up by the U.S. State Department as a "virtual embassy" hours after its launch in December. Washington has had no diplomatic presence in Tehran since its embassy was stormed in 1979, the year of the Islamic Revolution and has led the global push to isolate Iran.
"Iran's people have had to endure an ever-tightening stranglehold of censorship," Hague said.
"The blocking of our website is only a very small part of what Iranians undergo daily: millions of websites blocked, access to e-mail services denied, international television channels jammed, films and theatre productions closed down, books unpublished, traditional Persian literature rewritten and newspapers banned."
Many Iranians get around a government filter that blocks vast numbers of Western news and social media sites - including Facebook and Twitter - by using virtual private network, or VPN, software.