Turkey's plans to make adultery a crime could affect its chances of joining the European Union, EU enlargement commissioner Guenter Verheugen says.
Guenter Verheugen stressed the need for reforms during his visit
The bill, to be presented to parliament next week, may be seen as Islamic law entering Turkish law, he warned.
Prime Minister Recep Tayyip Erdogan, a devout Muslim, says the bill will help protect women from deception.
An EU report due out in October will assess Turkey's progress towards meeting EU membership criteria.
The adultery law is part of a package of sweeping changes to the penal code, which include the abolition of torture and the expansion of individual liberties. The changes are an effort to bring Turkey's legal code into line with European human rights legislation.
Mr Verheugen, who has spent the last few days touring Turkey, expressed his concerns in an interview with the Turkish Vatan newspaper.
"If Turkey tries to include crimes that are not in other countries' laws in its penal code, European Union countries could interpret this as Islamic law entering Turkish law,"
he told the paper.
He added that he was not "defending adultery", but said "Turkey should not give the
impression... that it is introducing Islamic elements into its legal system while engaged in a great project such as the EU".
Women's groups and liberal commentators have condemned the bill, saying it would be used against women and pushes the secular Muslim state closer to an Islamic legal model.
But the main opposition party says it will not challenge it, provided men face the same penalties as women.
The BBC's Virginia Gidley-Kitchin says adultery used to be illegal in Turkey until 1996, when the Constitutional Court struck the law down because it penalised women more than men.
Men were deemed to have been adulterous if they were involved in a long-term affair; but women could be charged if they were unfaithful only once.
Temel Karamollaoglu, a member of the Islamist Saadet party - which is more conservative than Mr Erdogan's governing AKP - says the law is necessary to protect the family and the society.
"At present adultery is accepted as a cause for divorce, and it is not accepted in society," he told the BBC. "The point is whether it should be punishable or not.
"We think that Turkey should join the EU, but not really accepting every detail in the moral value, not every aspect of European society at present. Countries may have different cultures. I accept European Union as a multi-cultural, multi-religious society."
Women's groups plan to demonstrate against the bill when it is presented to parliament on 14 September.
Canan Arin, of the Women's Rights Centre at the Istanbul Bar Association, says it is a violation of the constitution protecting individual's privacy.
"Everyone has the right to demand respect for his private and family life," she said.
Ms Arin fears the bill will work against women, as traditional women are reluctant to complain about their husbands.
"If they bring it, it will provoke honour killings more than ever," she said.
The BBC's Jonny Dymond in Istanbul says Mr Verheugen has given the impression of enjoying his visit but has stressed that it is the implementation of reforms that he is most interested in.
He stressed that the use of torture must be punished and he called for further cultural freedoms for Turkey's large Kurdish community.
According to our correspondent, it seems pretty clear from his more informal comments and demeanour that Mr Verheugen wants to give Turkey the kind of report which would boost its membership hopes.