Turkey's Constitutional Court on Tuesday declared invalid the first round of a controversial presidential election.
The court upheld an appeal from the secularist opposition that wants to stop the ruling Islamist-rooted AK Party's presidential candidate, Foreign Minister Abdullah Gul, becoming head of state.
"A decision was taken to stop the process," Hasim Kilic, deputy head of the top court, told a news conference. The court's rulings are binding and cannot be appealed against.
Prime Minister Tayyip Erdogan could now propose a different candidate for the top job, but most analysts say he is more likely to postpone the presidential poll and call a snap parliamentary election to help defuse tensions.
Turkey's financial markets and business community would prefer such a move, believing the pro-market AK Party would win another big majority in parliament with which to press ahead with reforms.
Under this scenario, outgoing President Ahmet Necdet Sezer would remain as interim head of state until a new parliament could choose his successor.
Parliament elects the president in Turkey for a seven-year term.
The secularist Republican People's Party (CHP) said last Friday's first round of voting in the presidential election was invalid because fewer than two thirds of deputies in the 550-seat parliament attended the session.
The government had argued that 184 deputies was sufficient for the vote to be valid.
The court decision means a planned second round of voting set for Wednesday will not now go ahead. The Constitutional Court's rulings are final and cannot be appealed against.
