The Vatican has clarified recent comments by Pope Benedict XVI on condoms, saying he did not mean they could be used to avoid pregnancy.
The Congregation of the Doctrine of the Faith said some analysts had misunderstood the remarks, made by the Pope in recently published interviews.
He said condoms could reduce the risk of HIV infection in certain cases, such as for a male prostitute.
The interviews were published in a book entitled Light of the World.
The Church's hard-line stance over contraception has led to the Vatican being heavily criticised for its position on the global Aids crisis. Some commentators suggested the comments represented a softening of this stance.
The Congregation of the Doctrine of the Faith (CDF) is the Vatican's moral watchdog, which Cardinal Josef Ratzinger led before he was elected Pope in 2005.
It stressed the Pope's logic was "in full conformity with the moral theological tradition of the church".
In a statement, the CDF said that he had not been talking about sex between a married couple or using condoms as a form of contraception.
"The idea that anyone could deduce from the words of Benedict XVI that it is somehow legitimate, in certain situations, to use condoms to avoid an unwanted pregnancy is completely arbitrary and is in no way justified either by his words or in his thought," said the statement.
It confirmed that the Pope's attitude towards homosexuality and artificial contraception had not shifted; other passages in the book reaffirm the Vatican's opposition to both, the CDF said.
Reaffirming that the Church considered prostitution "gravely immoral", the statement continued: "However, those involved in prostitution who are HIV positive and who seek to diminish the risk of contagion by the use of a condom may be taking the first step in respecting the life of another even if the evil of prostitution remains in all its gravity."
In Light of the World, when asked whether the Catholic Church was not opposed in principle to the use of condoms, the Pope had replied: "She [the Catholic Church] of course does not regard it as a real or moral solution, but, in this or that case, there can be nonetheless, in the intention of reducing the risk of infection, a first step in a movement toward a different way, a more human way, of living sexuality."
He cited the example of the use of condoms by male prostitutes as "a first step towards moralisation", even though condoms are "not really the way to deal with the evil of HIV infection".
The Pope's meaning had originally been questioned because the Italian translation of the book used the feminine form of the word for prostitute, whereas the original German used the masculine.
There was further confusion when Vatican spokesman Father Federico Lombardi said he had personally asked the Pope if there was a serious, important problem in the choice of the masculine over the feminine.
"He told me 'No'," Fr Lombardi said. "The problem is this... It's the first step of taking responsibility, of taking into consideration the risk of the life of another with whom you have a relationship."
"This is if you're a woman, a man, or a transsexual," he added.
Veteran Vatican analyst John Allen said what was at issue was not abstract moral teaching, but rather concrete pastoral application to a specific set of facts.
"If someone were to ask a Catholic priest, 'Is it okay to use a condom?' the answer is still supposed to be 'No'," said Mr Allen, a senior correspondent for the US-based National Catholic Reporter.
"Catholic teaching holds that to be fully consistent with God's plan, sexuality should occur only inside marriage and should be open to new life.
"If the question, however, is, 'I'm HIV positive and will have sex regardless of what the Church thinks, so is it better to use a condom to try to save lives?' the Pope has implied that a pastor might legitimately say 'Yes,' while still stressing that condoms ultimately are not, as Benedict says in his interview, a 'real or moral solution.'"
The book - Light of the World: The Pope, the Church and the Signs of the Times - is based on interviews that the Pope gave the German Catholic journalist, Peter Seewald, earlier this year.