Zagreb wrapped up accession preparations last month , paving the way for it to become the second Western Balkan state after Slovenia to join the EU, more than two decades after the bloody breakup of ex-Yugoslavia.
"We only have technicalities left. We hope in September we will meet in Zagreb and I will be able to hand over the final version of the treaty," Polish Prime Minister Donald Tusk told a news conference with his Croatian counterpart Jadranka Kosor.
"We expect the end of November or start of December to be the time when we are ready to sign it," Tusk said.
Poland holds the EU's six-month presidency until Dec. 31.
"Then we will have important events -- a referendum (in Croatia) and ratification (by the 27 current EU states) and I am sure that all these stages will happen according to our plan, making Croatia an EU member in 2013," Kosor said.
Zagreb is likely to join the EU formally in July 2013, a Polish presidency source told Reuters.
Croatia's accession negotiations dragged on for six years, hampered by EU concerns over slow judiciary reforms and Croatia's approach to its war-time past. The ex-Yugoslav state wrapped up its preparations to join the bloc last month, although its anti-graft efforts will continue to face EU scrutiny.
The EU hopes Zagreb's entry will encourage other countries in the region keen to join -- such as Serbia or Albania -- to pursue reforms as well as overcome the bloc's own "enlargement fatigue".