Commenting on the talks held at a working dinner at the summit of EU leaders, Oreskovic said that Cameron said that migration was the issue that tipped the scales in favour of Britain's exit from the EU.
"They wanted to have control so that they can decide for themselves who to let into the country, and that was possibly the key factor," said the Croatian PM.
He added that Cameron would not send an official notification to the EU on the launching of negotiations on exit conditions but that one would wait for a new leader of the Conservative Party and new Prime Minister to be elected, which was expected to happen on September 9.
Oreskovic said that Britons would like to divorce the EU but keep the conditions they had enjoyed until now, but that many told them they could not "have their cake and eat it too."
On Wednesday afternoon, the leaders of 27 EU countries will hold for the first time an informal meeting without representatives of Great Britain, to confirm their determination to continue together as a 27-member bloc.