Voters in Italy have roundly rejected radical plans to boost the powers of the prime minister and regions in a nationwide referendum.
PM Romano Prodi (left) is deeply opposed to the bill
According to final results, 61.7% of voters opposed the reforms, while 38.3% approved them.
The reforms were promoted by the previous centre-right government led by Silvio Berlusconi. He argued that Italian politics needed more stability.
The current centre-left government of Romano Prodi campaigned against them.
The BBC's David Willey in Rome says legislators are now likely to go back to the drawing board to decide how to ensure more stability in a country which has had 61 governments since 1945.
Just over half the 47 million registered voters turned out to cast their ballots in the two-day referendum.
Berlusconi's leadership of the opposition may be at risk
Mr Prodi was deeply opposed to the reform bill, which was sponsored by the populist Northern League party.
The Northern League leader, Umberto Bossi, had made the progression of the bill a prerequisite for his continued support of Mr Berlusconi.
He has raised the prospect that the Northern League will split from the centre-right coalition - a move that would undermine Mr Berlusconi's leadership of the opposition.
Under the reforms, the prime minister would be granted powers to dissolve parliament, appoint and dismiss ministers and determine the general direction of government policy.
These were all powers that were deliberately kept out of the hands of the prime minister in the 1948 constitution, as a way of preventing the emergence of another figure like fascist dictator Benito Mussolini.
KEY POINTS OF REFORM
Most radical shake-up since constitution introduced in 1948
Strengthens prime minister's powers, allowing him to dissolve parliament
Cuts president's powers
Twenty regions would get autonomy over education, health and local policing
Cuts number of representatives in both chambers
Parliament approved changes by a simple majority last November
The bill would also redefine the role of Italy's two parliamentary chambers.
The lower house - the Chamber of Deputies - would take the dominant role in matters of national interest like foreign policy, defence and immigration.
The upper house - the Senate - would become responsible for federal law.
But most significantly this bill would give greater autonomy to Italy's 20 regions. They would gain control over education, healthcare and law and order, and would win special representation in the nation's supreme court.
Some estimates say regional government would take control of around 40% of public expenditure.
Vannino Chiti, Mr Prodi's minister for institutional reforms, called the proposals "an awful mess".
"We would end up with 20 regional health systems, 20 regional school systems, and all sorts of divisions among Italians," he said.
The poorer regions in the south are hugely dependent on tax money that comes from the north, the BBC's Christian Fraser reports from Rome.
Two 2005 studies by groups looking at child health in Italy found that if the south were independent it would be the poorest of the 25 EU members in terms of per capita national income.
There were fears the constitutional changes could lead to disparities in the quality of public services, with teachers and health professionals deserting the south, preferring better wages and prospects in the north.
Another key argument against the reforms was the cost.
Italy urgently needs to cut its budget deficit. The proposals might add expensive layers of bureaucracy to a system of public administration where there is already plenty of waste, Christian Fraser reports.