Iran’s Supreme Leader issued a fatwa banning blank votes in the upcoming presidential elections, warning that the protest could "lead to the weakening of the Islamic regime".
Ayatollah Ali Khamenei's comment was a response to a question in a magazine issued by his office that asked the Supreme Leader what voters should do if unsure which candidate to vote for in upcoming presidential elections.
When asked whether leaving a ballot paper was permitted he responded: "Voting with a white paper, whatever the reason, if it leads to the weakening of the Islamic regime in Iran, is forbidden".
The response was a de-facto fatwa, a ruling on a point of Islamic law given by a recognised authority.
Khamenei had previously called for wide participation in the presidential elections, warning that staying away would mean doing the work of the "enemies of Islam".
Iranians are set to elect a successor to President Hassan Rouhani on 18 June amid widespread discontent over a deep economic and social crisis, which has been made worse by crippling US sanctions.
The opposition based outside Iran is running a campaign on social media networks calling for people to stay away from the polls, using hashtags in Farsi such as #NototheIslamicRepublic.