Italian PM Matteo Renzi has said he will resign after losing a key referendum on constitutional reform.
Mr Renzi had staked his future on a “Yes” vote, vowing to quit if voters rejected his plans to reduce the role of the country’s Senate and take back powers from regional authorities.
The result could plunge the country into political turmoil, with the effects felt on the markets and across the rest of Europe.
“The experience of my government ends here,” Mr Renzi said in a televised address to the nation after early results suggested he may have lost by as much as 20 points.
Mr Renzi, who came to power in 2014, said he took full responsibility for the “extraordinarily clear” defeat.
He will convene his cabinet for the last time on Monday afternoon and then hand in his resignation to President Sergio Mattarella.
A projection by the Piepoli Institute/IPR for state broadcaster RAI estimated 57-61% will vote “No”, compared to 39-43% for “Yes”.
The projection pointed to an even wider margin of defeat for Mr Renzi than was suggested by three exit polls released immediately after polls closed.
Opposition parties, who denounced the proposed changes as dangerous for democracy, were quick to call for Mr Renzi to go once the scale of the defeat became clear.
“Renzi is going to go and with him the powerful lobbies who were also defeated”, Renato Brunetta, the parliamentary leader of former prime minister Silvio Berlusconi’s centre-right Forza Italia party.
Matteo Salvini, the head of the Northern League party, earlier said Mr Renzi should quit “in the coming minutes”, and called for early elections.
Spearheaded by the populist Five Star Movement, the biggest rival to Mr Renzi’s Democratic party, the “No” campaign took advantage of the Mr Renzi’s declining popularity, a struggling economy and problems caused by tens of thousands of migrants arriving from Africa.
The vote is a major victory for Five Star leader Beppe Grillo, who urged Italians to follow their gut instincts.
But it is another blow to the European Union, which is struggling to overcome a number of crises and was keen for Mr Renzi to continue his reform drive.
Turbulence on the markets also looks inevitable.
Some analysts fear a deeper crisis of investor confidence that could derail a rescue scheme for Italy’s most indebted banks, triggering a wider financial crisis across the eurozone.
President Mattarella is now charged with brokering the appointment of a new government, or if that fails, ordering early elections.
Many analysts think the most likely outcome is that Mr Renzi’s administration will be replaced by a caretaker one dominated by his Democratic Party which will carry on until the next election, which must be held by spring 2018.