If you say δεν είναι πιθανό you you mean that it has zero possibility so "it's not possible" is ok I believe. If you wanted to say it's not likely (leaving some small possibility that it could happen) you would say δεν είναι πολύ πιθανό. (It's not very possible, literally), meaning that the odds are there, but low.
I had the same feeling as loukaki, but since I'm not a native Greek I asked some of my friends here in Greece what they thought, and they also said that δεν είναι πιθανό means 'it is not likely'.
On Linguee (http://www.linguee.com/english-greek/) you can find a list of translations of δεν είναι πιθανό (just paste and copy into the search field): it is either translated as 'it is not likely' or 'it is unlikely' - and not once as 'it is not possible' or 'it is impossible'.
If you check the sources you will see that these are translations made by official translators of the Eurpean Union. [Also, if you search on Linguee you will see that είναι πιθανό translates to 'it is likely', and 'it is impossible' always to δεν είναι δυνατό or είναι αδύνατο.]
It seems like the confusion stems from the phrase δεν υπάρχει πιθανότητα, which means 'there is no chance/possibility'.
[EDIT: I had answered with 'It is not likely' and it was rejected. Just now I tried with 'It is not probable' and it was accepted.]
@troll1995 [DL did not allow me to add to the last thread (probably too nested already)]. Like I said, I'm not a native Greek speaker. What can I say. I also assume that the translators/moderators here on DL are native Greeks. Maybe it's regional. Or maybe if I had presented my question with the right context (I just asked "How would you translate/explain δεν είναι πιθανό?) I would have gotten a different answer.
Yes, "it's not likely" means that there is still a chance that it will happen.
I just asked a friend of mine who is an English teacher and said that δεν είναι πιθανό=είναι απίθανο=it's not possible (because of the zero possibility thing). Πιθανός on its own is "likely". It might be regional though, to that I cannot answer. :) The way I know it/use it δεν είναι πιθανό=δεν ειναι δυνατό. It's not likely is δεν είναι πολύ πιθανό.
"If it is not probable" is accepted, "it is not likely" must be accepted as well, since probable and likely mean the same thing. Also "probable" is listed as one of the hint definitions of the word, so likely would fall under the meaning as well. And yes, not probable still allows some chance for the thing to happen.
When you say "it's not likely" do you mean that there is still a chance that will happen?
As a native English speaker this is my take on the matter:
Yes, there is still a chance, however "it's not likely" could also be used as a gentler/less blunt/less abrupt way of saying "it's not possible" or "it's impossible", for example, in a professional setting.