"The start of negotiations now is too early because the human-rights situation is dissatisfying," European People's Party chief Hans-Gert Poettering said on Friday after a meeting of his group in the EU assembly.
He said one possible alternative to full EU membership was a "privileged partnership" with the bloc.
"It's premature to begin negotiations now, and if there should be negotiations the large majority of our group [believe] they must be opened for different results," he said.
The European Commission is preparing to publish a report on 6 October expected to recommend that the EU start entry talks with Ankara. The report will form the basis of a decision by EU leaders due at a December summit.
Critics argue that Turkey is simply too different culturally, economically - and in sheer terms of population - to be successfully integrated anytime soon into the EU. In 2004, the EU witnessed its largest expansion to date, to 25 states.
Edmund Stoiber, a senior member of Germany's conservative opposition, defended his party's opposition to Turkish entry into the EU, saying the bloc could not cope with such a large new member.
"Our concern is that Europe is simply not in the position to take in a country such as Turkey," Stoiber, Bavarian Prime minister and Christian Social Union (CSU) leader, told a meeting of the foreign press association.
Critics say Turkey is too different
culturally to be part of the EU
Stoiber said Turkey was and would remain an important partner for Germany and the EU and said the Europeans and Turkey should work towards developing a "privileged partnership" instead.
Hostility towards Turkey's attempt has fuelled charges that the European Union is a "Christian club", a sentiment Stoiber did not entirely reject.
"There is a basic code from Finland to Portugal. Turkey has a different history. But this is not the largest obstacle. How much would it cost?" Stoiber asked, adding that aid for Turkish agriculture alone would reach 11 billion euros.
Meanwhile, Jose Manuel Durao Barroso, European Commission chief-in-waiting, said the EU should send Turkey a positive signal on its chances of joining the bloc as long as Ankara met the necessary criteria.
"I think it is important to keep our credibility. If we said that Turkey could become a member or could [start] negotiating with us if they fulfilled certain criteria, I think now we should give a positive signal," Barroso said following a meeting with Finnish Prime Minister Matti Vanhanen.
"Our concern is that Europe is simply not in the position to take in a country such as Turkey"
German opposition politician
"We cannot change our criteria to satisfy any country wanting to become a member, but if a country like Turkey accepts our criteria, we should welcome them. It will improve our market, our democracy and build bridges for peace."