By Sarah Rainsford
BBC News, Istanbul
Turkey's president has vetoed a bill intended to bring forward a referendum on whether the Turkish people should directly elect their next president.
President Sezer first supported the plan but has now used a veto
The ruling AK Party initiated the reform after failing to get its own candidate elected by parliament.
Opposition MPs claimed the candidate, a former Islamist, was a threat to Turkey's secular system.
MPs then voted to allow a referendum on the proposed changes to be held on the day of a general election in July.
But President Ahmet Necdet Sezer has now used his veto, making it increasingly unlikely the reform will ever become reality.
Reforms in danger
The government wanted this bill passed, so Turkey could hold a referendum on introducing direct elections for president at the same time as the general election next month.
The ruling AK Party (AKP) believes that is its best chance to get its candidate into office.
But, as expected, Mr Sezer has moved to prevent that - and now the government's entire constitutional reform package looks close to collapse.
The AKP began pushing to have the people choose the Turkish president when its own candidate for the post, Foreign Minister Abdullah
Gul, was blocked by parliament.
Turkey's staunchly secular main opposition party claimed the former Islamist was a threat to the system.
The AKP calls that undemocratic, and insists its candidate would win a popular ballot.
But the party's opponents are clearly reluctant to test that theory in practice.
The opposition Republican People's Party (CHP) has already applied to the constitutional court to overturn the reforms on a technicality.
And now the president has lodged his own complaint with the court.
The judges may give their verdict on Tuesday. If they rule against the AKP most believe the party will abandon its reform effort for now, to focus on the general election.
The opposition is battling to weaken the party's control of parliament then and simultaneously prevent any AKP candidate becoming president.