Pope Benedict XVI's first encyclical, expected in the next few days, warns believers not to confuse love with lust or degrade it "to mere sex".
It is Pope Benedict's first encyclical
The encyclical, a papal letter to bishops that sets out Roman Catholic policy, discusses the relationship between "eros", or erotic love, and "agape", a Greek word referring to unconditional, spiritual and selfless love.
"It is not totally negative on eros," a Vatican source said. "It argues that eros under the right circumstances is OK."
But the Pope issues a warning in the document, entitled Deus Caritas Est (God is Love), that eros risks being "degraded to mere sex" if it is not balanced with spiritual or divine love founded on the teachings of Jesus.
John Allen, a columnist with the National Catholic Reporter and one of the most respected Vatican watchers, said: "The Pope wants to make sure that everything he does is grounded in fundamentals in terms of objective truth.
"The encyclical is his attempt at being a compassionate conservative. In his mind, you can't really be free and happy unless you accept God's plan for human life."
Whe he was still Cardinal Joseph Ratzinger, the Pope was known as a staunch traditionalist whose election as pontiff filled liberal Catholics with dismay. While the encyclical focuses on sex, it is likely to be a good deal less controversial than the Vatican's recent instruction banning homosexuals from the priesthood.
Although the instruction is a much less important form of Vatican communication, it has infuriated Christian gay activists who see it as discriminatory.
In explaining his views on love and sex in the encyclical, the Pope quotes from biblical writings, encyclicals written by his predecessors and the works of philosophers such as the 17th century French thinker René Descartes.
He wrote the first part himself during his holiday in the Alps last summer. The second part, dedicated to the theme of charity, draws on the work of theologians working under Pope John Paul II, who died last April.
Italian newspapers reported the encyclical as saying that even in "more just societies" Christians should do charitable works, not just for the benefit of others but for their own good.
The Vatican declined to confirm that the encyclical would appear on Friday. But the magazine Famiglia Cristiana is to publish it as a special supplement on Jan 25.
The document was originally due out last December but was delayed as cardinals and senior theologians pored over every word.
Pope Benedict's first encyclical could prove a profitable source of income for the Vatican. The leaking of its contents coincide with news that the Vatican is to transfer copyright on papal texts to its own publishing house, which will then charge others wishing to publish them.
The introduction of Vatican publishing rights is one of the new Pope's first important administrative acts. A major source of controversy between the Vatican and publishers wishing to reprint papal texts will be the Vatican's desire to charge rights retroactively on any papal texts of the past 50 years.
The last pope published 2,770 titles under his name in English, 1,000 in Spanish and 330 in Italian, plus titles in other languages.
When Pope Benedict was still a cardinal, he published hundreds of texts, especially in his native Germany, with publishers having already acquired the rights. They could now face demands for hefty back-payments.