Turkey's governing AK Party is standing trial for anti-secular activities, accused of trying to turn the country into an Islamic state by stealth.
The BBC's Pam O'Toole explores the background to a case which threatens to bring down the government.
Who is behind the move against the AKP?
The case to close the AKP (Justice and Development Party) has been brought by Abdurrahman Yalcinkaya, chief prosecutor of Turkey's Court of Appeal.
Like many members of Turkey's judiciary, he is regarded as a staunch Kemalist, determined to defend the secular principles enshrined in modern Turkey's constitution by its founding father, Mustafa Kemal Ataturk, in the 1920s.
Mr Yalcinkaya argues that the ruling party represents a threat to those principles and that it is trying to turn Turkey into an Islamic state by stealth.
He has the backing of large parts of the Turkish establishment, including the main secular opposition party - the CHP - which was originally founded by Ataturk and also the powerful army, which sees itself as the guardian of Ataturk's legacy.
What does the AKP say?
The AKP alleges that the current case is politically motivated and undemocratic, launched by an out-of-touch secular elite trying to prevent its power slipping away in a changing society.
The party's supporters allege the military is trying to use the court to close the party in what they term "a judicial coup".
Since it was formed seven years ago by moderates and modernists from a previously banned pro-Islamist party, the AKP has won convincing victories in two successive general elections.
In parliamentary polls last year it won 47% of the vote from a broad section of Turkish society.
It insists that it has renounced political Islam and points to the outward looking, pro-Western policies it has adopted during five years in power, including democratisation reforms aimed at easing Turkey's path to EU membership.
Is it common to ban a political party in Turkey?
Yes, it is relatively common. But this is the first time that a closure case has been brought against a ruling party with a huge parliamentary majority.
More than 20 political parties have been banned since 1960. Most of them were either pro-Islamist or pro-Kurdish in outlook and were closed after being accused of posing a threat to the united, secular, Turkish state envisaged in the constitution.
In the late 1990s one pro-Islamist party, the Welfare Party, briefly held power as part of a coalition government.
But it was later banned and its Prime Minister, Necmettin Erbakan, was eased from office, following pressure from the military, in a so-called "soft coup".
A number of pro-Kurdish parties have also been banned after being accused of having links with Turkey's armed Kurdish militant group, the PKK, which is regarded by Ankara, the US and the EU as a terrorist organisation.
The current main pro-Kurdish party, the Democratic Society Party, or DTP, which has 21 MPs in parliament, is also currently threatened with closure by the Constitutional Court.
What can the AKP do, faced with this banning threat?
The party has mounted a determined defence of its position both inside the Constitutional Court and with the Turkish public.
It has also been talking about introducing constitutional reforms to make it more difficult to ban parties in the future.
If the AKP is eventually closed, one option would be for it to do what many other outlawed parties have done in the past - re-form under another name and carry on business as usual.
The party's MPs could possibly continue to serve in parliament as independents until a new party was created.
However, the Constitutional Court is also being asked to ban more than 70 of the party's senior leaders from politics for up to five years.
They include the prime minister, five ministers and more than 30 other MPs.
If that were to happen, elections might have to be held to replace those disqualified from parliament.
There has been also been speculation that if the AKP is closed and a number of its leaders disqualified from politics, what remains of the party might decide to call snap general elections anyway in order to get a new mandate from Turkish voters.
But there are a number of other scenarios which are also possible, depending on what the court decides to do. Exactly how the AKP's leadership will respond will only become clear after the court has made its ruling.
What other effects might a ban have?
There are fears that it could cause political and economic instability. The closure case has already caused uncertainty in the Turkish stock market and nervousness among foreign investors.
There has been speculation that a ban could also harm Ankara's bid to join the European Union.
The EU has criticised the closure case, saying such political issues should be debated in parliament and decided through the ballot box, not in the courts.
Meanwhile, opinion polls suggest that many ordinary Turks are opposed to the AKP being banned - and that could have internal political repercussions.
When Turkey's secular establishment tried to block the AKP candidate from being elected president last year, voters responded by increasing their support for the party in general elections held last summer.