ROME — Prime Minister Prodi has called on Roman Catholic priests to help him battle Italy’s widespread tax evasion by invoking the seventh commandment — thou shalt not steal.

Mr. Prodi made the appeal in an interview this week with Italian religious affairs weekly Famiglia Cristiana. His comments sparked fierce criticism that he is blurring the lines between church and state, and on Thursday Mr. Prodi defended himself in a front-page letter to Italy’s top daily.

“A third of Italians heavily evade taxes,” Mr. Prodi told Famiglia Cristiana. “To change this mind-set, everybody, starting with the teachers, must do their part , school and church included.”

“Why, when I go to Mass, is this issue, which is ethically charged, almost never touched upon in the homilies?” Mr. Prodi asked.

Tax evasion is a chronic problem in Italy. The government has launched a crackdown and claims to have recovered $16.4 billion in unpaid taxes last year. But according to government estimates, unpaid taxes, including income from the country’s black-market economy, are equal to 27% of Italy’s gross domestic product.

In Thursday’s letter to Corriere della Sera, Mr. Prodi said tax evasion is “the main reason why we have both overly high taxes for honest people and a heavy deficit in the state’s balance.” He said that it was up to the ruling class to set a good example. But, he added, “if memory serves, St. Paul exhorted [citizens] to obey the authority.”

“Instead of telling parish priests what to do or not to do, Mr. Prodi should simply lower taxes,” Daniele Capezzone, a lawmaker from the small anti-Vatican Radical Party, said.