Matteo Renzi

Mr Renzi, 42, won more than 70% of the vote in Sunday's primary among party supporters, leaving two other contenders far behind.

The victory propels him back to the forefront of national politics.

Mr Renzi resigned as prime minister and party leader after his constitutional reform plan was rejected in a national referendum in December.

"This is an extraordinary responsibility. Heartfelt thanks to the women and men who believe in Italy," he wrote on Instagram on Sunday.

Mr Renzi formally resigned on Monday after his defeat in a constitutional referendum a day earlier, but the president requested the delay.

President Sergio Mattarella will later have to either choose a new prime minister or call early elections.

European leaders have been playing down the risks of fallout from the crisis.

Matteo Renzi warned that leaving the EU would be a "very difficult process" - but the problems could be solved only after the UK began the exit procedure.

He said the Brexit vote had been "a bad decision" but had to be respected.

Mr Renzi said he was ready to work with UK PM Theresa May to build the "best alliance" between the UK and the EU.

Mr Renzi said he had been shocked and saddened by the EU referendum result, but repeated Mrs May's vow that "Brexit is Brexit", saying democracy had to be respected.