By Oana Lungescu
BBC correspondent in Brussels
Negotiations enabling Turkey to join the European Union would take at least a decade, according to two reports obtained by the BBC.
It may take time but there is light on the horizon for Turkey's EU bid
The EU documents reveal that the country has made significant progress on human rights.
They also show the cost of Turkey's EU's membership to be as much as that of the 10 mostly former communist countries that joined the EU this year.
A positive decision on membership talks is expected next week.
The closely-guarded reports amount to over 200 pages.
The tone of the overall assessment is positive, highlighting significant progress in all the key areas that will determine whether Turkey can begin entry talks.
The country, the reports say, has made progress on everything from the abolition of the death penalty to strengthening the fight against torture.
Turkey's geostrategic role also lays heavy in the balance.
A separate impact study on Turkish membership says that, in the aftermath of the 11 September attacks, the successful inclusion of Turkey would give clear evidence to the Muslim world that their religious beliefs are compatible with EU values such as democracy and the rule of law.
But the commission also makes clear that entry talks could last well into the next decade and that Turkey may not be eligible for full EU subsidies until 2025.
It would then receive between 17bn and 28bn euros (£11bn - £19bn) from EU coffers.
Such figures, though, are highly uncertain, the commission argues, because no one knows what the EU and Turkey will look like in 20 years' time.
The accession of Turkey would be challenging both for the EU and Turkey, the impact study concludes, but, if well managed, it would offer important opportunities for both.
On Wednesday, the European Commission is expected to announce a shift in its expansion strategy to ease fears about Turkey's eventual accession.
The country would be monitored more regularly and intensely than any other candidate before, with an explicit warning that membership talks could be delayed, or even halted, if it fails to continue reforms.
Making it clear that full membership is not a foregone conclusion would enable EU leaders to take one of their most controversial decisions ever at their December summit and finally announce a date for opening negotiations with Turkey.