By Philip Pullella

VATICAN CITY (Reuters) - Pope Francis on Monday encouraged the people everyone loves to hate - tax collectors - telling them that while they will never win popularity contests, they were vital for the functioning of a fair society.

Your work appears to be thankless ..., he told a delegation from the Agenzia delle Entrate, Italy's revenue agency, acknowledging that taxation was often seen as putting your hands in other people's pockets.

But Francis said everyone had to pay their fair share of taxes, particularly the wealthy, so that the weakest members of society were not crushed by the most powerful people.

In reality, taxation is a sign of legality and justice, he said.

He praised the honesty of those who pay their taxes, denouncing tax evasion and the underground, or off-the-books, economy.

In Italy, where a comedian once joked that tax evasion is the country's most popular sport after soccer, an estimated more than 100 billion euros a year is lost to tax evasion, according to recent statistics.

Officials also estimate that Italy's underground economy - without contracts, social security contributions or taxes deducted - is worth about 200 billion euros a year, or about 11% of Gross Domestic Product (GDP).

Francis also told the tax collectors that while they may not be showered with affection on earth, they have a patron saint in heaven. He reminded them that St. Matthew the Apostle was a publican, or tax collector in Roman times, before he decided to follow Jesus.

(Reporting by Philip Pullella; additional reporting by Crispian Balmer and Giuseppe Fonte; Editing by William Maclean)