SARAJEVO (Reuters) - Bosnia's first census since its 1992-95 war was postponed on Wednesday until October after regional governments failed to agree on how to conduct it.

Something as routine as counting citizens is very sensitive in Bosnia, where ethnic and religious divides were at the heart of wars in the 1990s when some 100,000 people were killed and 2 million displaced during the collapse of Yugoslavia.

Bosnia's regions are linked by a weak central government - dogged by sensitive issues around ethnicity, religion and language - with development and reforms often hostage to ethnic politicking and conflicting visions of the nation's future.

Bosnia's Muslims are campaigning to make sure members of their ethnic group declare themselves as such in the census, fearing otherwise the results will cement the effects of wartime ethnic cleansing, diminishing the so-called Bosniaks' influence.

The government cited among the reasons for the delay: data exchange between regional agencies, revision of questionaires, training of field staff and registering addresses.

The census, originally planned for April this year and now due to be conducted between October 1 and October 15, may have ramifications for the complex system of ethnic power-sharing enshrined in the Dayton peace treaty that ended the fighting.

Rival Serb, Muslim and Croat political leaders have long been at odds over how to hold the census, but finally reached agreement in February last year under pressure from the European Union, which Bosnia wants to one day join.

EU officials had warned last month that a lack of political will to address census issues quickly was likely to force a postponement. The bloc needs up-to-date population data for its future members in order to better plan financial aid.

Dayton split Bosnia into two highly autonomous, ethnically-based regions: the Federation, dominated by Muslims and Croats, and the Serb Republic, dominated by Serbs.

The country's last census was carried out in 1991 when its population was 4.4 million: 43.7 percent Muslims, 31.4 percent Serbs and 17.3 percent Croats.

(Reporting by Maja Zuvela; Editing by Louise Ireland and Matt Robinson)