The new division into Adriatic, Pannonian and Northwestern Croatia was drawn up by the State Bureau of Statistics and approved by EUROSTAT.
HNS leaders Vesna Pusic and Radimir Cacic said the decision to include Zagreb into a region with five northwestern counties could have far-reaching consequences for the development of those counties.
Zagreb's GDP is twice as high as in those counties, near the European average and raises the average of the entire region.
The HNS said the entire region would reach the European average GDP in five years because of Zagreb and would no longer be entitled to assistance from European funds.
"Zagreb will get nothing out of that, while Croatia will lose one of the most important prerequisites for a balanced development," said Pusic, advocating that Zagreb should be declared a separate statistical region given that it meets the requirements for that.
Pusic recalled that parliament decided on the division of Croatia into five statistical regions in June 2003 and that this decision was neither repealed or put out of force.
She said the HNS wanted the prime minister to say if the parliamentary decision was ignored or if the government rejected it without putting it to parliament.
Cacic said the division into three regions showed the government was incapable of defending national interests, and that northwestern Croatia would suffer the most.
Pusic and Cacic said they would ask that parliament discuss this issue.