"It will not be easy for Turkey to swallow all that we have written," the enlargement commissioner said of the report to be published on Wednesday.
Details of the report emerged in the European media on Friday.
"According to me, Turkey would be able to join in 2015 at the earliest," Verheugen told the German newspaper Bild am Sonntag.
The report by the EU executive, a copy of which AFP has seen, listed several areas where Turkey has shown "a substantial institutional convergence ... towards European standards".
But it also highlights shortcomings in Turkey's EU preparations, including continuing reports of torture.
EU leaders will use the report at their 17 December summit to decide whether to start accession talks with Turkey.
"Although torture is no longer systematic, numerous cases of
torture - and in particular, ill-treatment - still continue to occur and further efforts will be required to eradicate such practice," the report said.
"In the end, it will be the European Parliament and the parliaments of member states that will decide" whether to admit Turkey, Verheugen told the paper.
"It will not be easy for Turkey to swallow all that we have written"
EU enlargement chief
"Each country can stop membership negotiations at any moment. There is nothing automatic or definitive" about agreeing to start entry talks, he said.
"Any decision that could be perceived by Turkey as a refusal
could spell the end of the reform process in that country," he
"The entire region would certainly become less stable as a
result. And in the long term it could even be a threat to Europe."
Criteria not met
Meanwhile, the EU's foreign policy chief Javier Solana said in a
separate interview with Bild newspaper on Saturday that Turkey had not yet met the criteria required to become an EU member.
"There must be an end to torture, minorities must be able to
enjoy full rights, and the media must be able to inform freely,"
"Ankara holds the key. If Turkey is determined to join the EU,
it must pursue the course of reforms that will enable it to do so," he said.
Turkey has been seeking EU membership since 1963, becoming an official candidate in 1999. But the bid has been controversial, with many existing EU states questioning whether the largely Muslim nation of 70 million could easily fit into the bloc.