By Gavin Jones

ROME (Reuters) - The number of people living in poverty in Italy hit its highest level for a decade in 2015, data showed on Thursday, in a report that could hurt centre-left Prime Minister Matteo Renzi.

Those living in "absolute poverty" rose to 4.6 million last year, or 7.6 percent of the population, up from 6.8 percent in 2014 and the highest since current records began in 2005, national statistics bureau ISTAT reported.

Renzi, who came to power in February 2014 promising to kickstart a chronically stagnant economy, has seen his popularity ratings fall steeply over the last year as economic and job growth has remained listless.

ISTAT's annual poverty report jars with his constantly upbeat message made up of tweets and hashtags like "Italy is back" and "We're turning Italy around."

The report is particularly sensitive because a common criticism of the 41-year-old prime minister is that he has taken his traditionally centre-left Democratic Party too far to the right and lost touch with the working class and poor.

ISTAT defines absolute poverty as the condition of those who are unable to buy goods and services "essential to avoid grave forms of social exclusion".

In the underdeveloped south of Italy, 10 percent of people were living in absolute poverty, up from 9 percent in 2014, ISTAT said. That compared with 6.7 percent in the north, up from 5.7 percent.

The situation was deteriorating for workers as well as the unemployed, with 6.1 percent of families living in poverty despite having at least one member in work. The figure rose to 11.7 percent if the breadwinner was a factory worker.

Renzi faces a crucial referendum in the autumn on a plan to reduce the powers of the Senate and streamline parliamentary procedures, and has repeatedly said he will resign if he loses.

ISTAT said Italians living in "relative poverty," or those whose disposable income is less than around half the national average, also rose in 2015 to 8.3 million people, or 13.7 percent of the population. That was up from 12.9 percent in 2014 and the highest since current records began in 1997.

(Editing by Catherine Evans)