SARAJEVO (Reuters) - The number of Serbs in Bosnia has fallen and the overall population in its two autonomous regions has dropped nearly 20 percent to just over 3.5 million, according to the long-awaited results of a disputed census, the first since the 1990s war.

The 2013 census is seen as a vital tool for economic and social planning, but Bosnian Serbs fear the count could have ramifications for the delicate system of power-sharing set in place by a U.S.-brokered peace deal in 1995.

A dispute, centering on how the census defines who lives in the Serb-dominated Serb Republic and the Federation shared by Croats and Muslim Bosniaks, has held up results for two years.

Serb Republic leaders fear the results of the census, conducted by the state statistics agency, is likely to reduce the number of Serbs allowed to work in Bosnia's regional and state governments and armed forces.

Results showed that half the population are Bosniaks or Bosnian Muslims, up from 43.5 percent in 1991, when the last census was held just before Yugoslavia's fall and the war in which 100,000 people were killed and two million displaced.

Bosnian Serbs make up 30.8 percent of the population, down from 31.2 percent in 1991, while the Croats account for 15.4 percent, down from 17.4 percent in 1991. The remainder are ethnic minorities, others and those undeclared.

The European Union has been urging Bosnia to start publishing population data by July 1 if it wants Brussels to consider the membership application it submitted in February.

A lack of data has hampered the development of economic, demographic and social strategies and prevented authorities from providing reliable per capita data, needed for assessing purchasing power and education levels among other issues.

(Reporting by Maja Zuvela; Editing by Giles Elgood and Louise Ireland)