The refusal by Iran's Guardian Council to approve hundreds of reformist candidates in the parliamentary elections on 20 February has provoked a political crisis.
What lies behind this decision?
This move is generally seen as part of the power struggle in Iran between the conservatives who want to maintain a strict Islamic approach and reformers, backed by the elected government, who want greater liberalisation.
Guardian Council is led by Ayatollah Jannati (centre)
Reformers control the parliament, the Majlis, but under Iran's constitution, a series of appointed supervisory bodies have the ultimate say and these are in the hands of the conservatives.
Iran is about to mark the 25th anniversary of the Islamic revolution which threw out the Shah. It may be that the conservatives felt that this was a good moment to try to prevent further domination of the parliament by reformers after the elections.
BBC regional analyst Sadeq Saba suggests that the conservatives were emboldened to disqualify so many because they reckoned that the man in the street would not risk his life by supporting the reformists.
Who has been disqualified?
In its first ruling on 11 January, the Guardian Council disqualified 3,600 of about 8,000 candidates overall.
It reinstated about a third of these after pressure from the reformist President Mohammad Khatami and the intervention of the conservative spiritual leader Ayatollah Ali Khamenei.
The banned list includes more than 80 sitting members of the Majlis, all of them reformers.
One of them is Mohammad Reza Khatami, head of the Islamic Iran Participation Front, the main reform party. He is also brother of the Iranian president.
MPs who recently wrote to Ayatollah Khamenei urging him to allow greater freedom are on the list.
Two women activists, Fatima Haqiqatjou and Elaheh Koulaee were also disqualified.
What is the Guardian Council?
The Guardian Council is a supervisory body which has the power to vet candidates for parliament, the presidency and the Assembly of Experts (which chooses the Supreme Leader) and to reject legislation not considered to conform to Islamic principles.
It has 12 members. Six are clerics chosen by the Supreme Leader and six are Islamic lawyers appointed by parliament.
The Council is led by the conservative Ayatollah Ahmad Jannati. He recently called for Iran to leave the Nuclear Non-Proliferation Treaty. He also accused the United States of trying to make political capital out of the Bam earthquake but said that Iran had given the Americans "a slap in the face."
What has the been the reaction of the reformers?
After a number of threats, reformist members of parliament resigned en masse over the move.
The 109 letters of resignation tendered are believed to include the 80 sitting MPs banned by the Council.
Interior Minister Abdolvahed Moussavi Lari, who is in charge of organising the poll, has warned that it will be "out of the question" to hold elections.
But the Guardian Council has rejected his request to postpone it.
The reformers see the move as an attempt by hardliners to block the path of liberalisation. Their attempts to pass laws have been frustrated by the Guardian Council.
One battle has been over the age of marriage which the parliament wants to raise from 9 to 13 for girls and from 14 to 15 for boys. The Council says that the lower ages should stay, as marriage is a good way of countering "immorality" among teenagers.
What about the position of President Khatami?
He is a reformer, too, and he has criticised the Council's decision. He wants the issue resolved through negotiations, and has said that disqualified candidates should appeal against the decision.
It is also possible that he will consult the Council of Expediency, a body set up in 1988 to mediate in disputes between Parliament and the Guardian Council. It, too, is seen as a mainly conservative body.
But he has warned that his whole government may resign if thousands of fellow reformists remain barred.
How might this affect Iran's external relations?
The battle for control of Iran has important international implications. Recently, Iran accepted the demands of the UN nuclear watchdog, the International Atomic Energy Agency, for stricter inspection of its nuclear energy programme. The United States is also hinting at dialogue with Iran, something the EU has already begun.
If hardliners regain control of the whole of Iran's complex government structure, such openings to the outside world may cease or slow down.
And Iran's attitude towards developments in Iraq could be affected.