The commission says there is resentment in Turkey at the slow pace
The European Union is in danger of breaking its promise that Turkey will eventually be granted membership, an influential group has warned.
The Independent Commission on Turkey accuses some European leaders of trying to derail Turkey's membership bid.
Behind the carefully balanced language of diplomacy is a hard hitting report, a BBC correspondent says.
It points the finger most firmly at France, whose President Nicolas Sarkozy is strongly opposed to Turkish entry.
"France has publicly declared that it will not allow five key areas of the negotiations to go forward, specifically because the current French leadership opposes Turkish accession," the report says.
The Independent Commission on Turkey is made up of senior European politicians and academics, and is led by the former Finnish President and Nobel Laureate Martti Ahtisaari.
Its report says negative statements from some leaders, and efforts to substitute full membership for some other kind of privileged partnership or special relationship, are putting the EU's credibility at stake.
"In 1999... we said that Turkey is a candidate state destined to join the union on the basis of the same criteria as apply to other candidate states. So it's the credibility of the EU [at stake]," Mr Ahtisaari said.
France, Germany and Austria are among the countries that object to Turkey having full membership of the EU, proposing instead a privileged partnership.
The report says a vicious circle has developed.
"Fierce opposition from some European politicians combined with growing public resistance to further EU enlargement in turn has deepened resentment in Turkey and slowed the necessary reforms," the report says.
It acknowledges that some serious obstacles remain to Turkish membership - not least, the impasse over the divided island of Cyprus.
Solving that frozen conflict would boost Turkey's membership bid - but the commission warns that talks are running short of time.