By John Geddie
LONDON (Reuters) - Italian bond yields were set for their biggest daily rise this year, leading a widespread bond sell-off, after Prime Minister Matteo Renzi suggested on Monday he may not stay on if he loses next month's referendum on constitutional reform.
Underlying bets that the policies of U.S. President-elect Donald Trump could push up inflation are feeding a global bond rout, but Italy stands out as one of several euro zone countries facing political tests in the coming months.
Renzi has recently declined to comment on earlier pledges to resign in the case of a defeat, saying discussion of his own future deflected attention from the merits of the reform.
But asked in a radio interview what he would do if the 'no' vote won in the Dec. 4 referendum, he said: "If I have to stay on in parliament and do what everyone else has done before me, that is, to scrape by and just float there, that does not suit me."
Polls show the 'no' vote firmly in the lead. Trump's unexpected victory in the U.S. election could make it even harder for Renzi to win, by bolstering support for his populist rivals in the 5-Star Movement.
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"If you talk to Italian investors, a lot of them will say that nobody else other than Renzi can govern in Italy. This is the reason why there was a widely-held view that Renzi, in the end, would stay on," Natixis' fixed income strategist Cyril Regnat said.
Italian 10-year yields rose 18 basis points to 2.15 percent, their highest since September 2015, as they stayed on track for their biggest daily rise since December 2015.
Thirty-year equivalents rose around 20 basis points to 3.34 percent, their highest since July 2015, and also on track for their biggest daily rise of the year.
The gap between 10-year Italian yields and benchmark German equivalents hit its widest since October 2014.
The referendum has also widened divisions in Renzi's Democratic Party (PD). Political commentators have speculated that if he does step down after a 'no' vote, he would then push for early elections in 2017.
That raises risks of a populist backlash in Italy, like the one that propelled Trump into power and Britain on the way out of the European Union. Upcoming votes in Austria, the Netherlands and France could also see major changes in the political landscape.
More broadly, investors are reasoning that the value of bonds will be eroded over time if Trump's promises of heavy infrastructure investment and deep corporate tax cuts boost global inflation.
German 30-year bond yields rose some 10 bps to break through 1 percent for the first time in more than six months on Monday, while U.S. 30-year yields topped 3 percent to hit the highest level seen this year.
A key market gauge of euro zone inflation -- the five-year, five-year forward rate -- climbed 3 bps to hit 1.59 percent, its highest since January.
"It is a continuation of this recent trend. There are still these expectations that inflation could go up if the U.S. takes a more expansionary fiscal stance," DZ Bank strategist Daniel Lenz said.
It is too early to say what measures Trump's administration will push or how it will fund them, but Republican control of both houses of Congress gives him great flexibility.
(Editing by Mark Trevelyan)