Iranian newspapers have been banned from publishing the names or photos of the leaders of Iran's green movement, according to a confidential governmental ruling revealed by an opposition website.

The move is part of a new round of censorship, which follows the recent closure of a newspaper and the suspension of two magazines.

The ruling, issued by Iran's ministry of culture and Islamic guidance on 18 August, was stamped "top secret" and "urgent". It was addressed to the editors of newspapers and news agencies in Iran, and bans them from publishing any news about the defeated presidential candidates in last summer's disputed election and current opposition leaders, Mir Hossein Mousavi and Mehdi Karroubi, and the former reformist president, Mohammad Khatami.

The opposition website irangreenvoice.com has published a copy of the letter, which reads: "Keeping the society and the public opinion calm is the main responsibility of the media. Security officials have considerations about publishing news, photos and speeches of Mr Mir Hossein Mousavi, Mehdi Karroubi and Mohammad Khatami, therefore according to the clause 2 of the article 5 of the press code publishing news, photos and reports about the these people are prohibited."

An Iranian journalist who works for a government paper, and asked not to be identified, told the Guardian: "Soon after the election last year, those papers which insisted on publishing news or reports about the opposition leaders were all closed down , so after a while an unwritten ruling overshadowed the media in Iran. Self-censorship meant no journalist even dared to utter the names of the opposition leaders to their editors, let alone publishing any news about them."

According to the journalist, the term "leaders of sedition" which was initially used by Iran's supreme leader, Ayatollah Ali Khamenei to denounce the opposition leaders, was allowed to be published in reference to the names of the leaders instead of directly mentioning them. The supreme leader, who has the ultimate power in Iran, has never denounced the opposition leaders directly by addressing them with their names.

"For us any news about the green movement is as if it has never happened ... It's shocking to know that even according to the current Iranian constitution, censorship is prohibited and the cultural ministry is not in the position to send such letters, but there's no way for us to complain. Once anyone complains, the paper will be closed down in no time," he said.

Last week, Iran also closed down Asia, a financial newspaper and suspended the permission for publication of two magazines, Sepidar and Parastoo.

Since the disputed election in June, Iran has shut eight newspapers, including Etemaad, Iran's most prominent reformist paper, and has imprisoned more than 100 journalists and bloggers. In Iran, according to a report by the New York-based Committee to Protect Journalists (CPJ) at least 65 remain in jail – more than any country has imprisoned since 1996. Almost all opposition newspapers are closed down and access to their websites are blocked.