By Gordana Katana

BANJA LUKA, Bosnia (Reuters) - Bosnia's Serbs said on Friday they would hold a referendum to set their Statehood Day on the divisive date of Jan. 9 - putting them on a collision course with the top court and other groups who say the celebration and its timing exclude them.

The Balkan country's Serbs hang out flags and broadcast Orthodox Christian ceremonies on the anniversary of the day in 1992 when they declared independence from Bosnia before a three-year war that claimed 100,000 lives.

Other groups say the celebrations strain the precarious unity of the country formed after that war - a state made up of two parts, the autonomous Serb Republic and the Bosniak-Croat Federation.

Bosnia's constitutional court ruled in 2015 the date should be changed as it coincided with an Orthodox Christian holiday and was therefore seen to exclude Roman Catholic Croats and Muslim Bosniaks who also live in the Serb Republic.

But the Serb Republic's parliament voted on Friday to hold a referendum within its territory on Sept. 25 on when to hold the celebrations, challenging the authority of the top national court whose rulings are supposed to be final and binding.

"Continuation in marking the Jan, 9 as Statehood Day, and the referendum to be held to this end, is the only possible response and a way for the existence and survival of the Serbs in Bosnia," Serb Republic parliament deputy speaker, Nenad Stevandic, said.

Bosniak lawmakers walked out of the session, saying the decision violated the constitution and the Dayton peace accords that ended the 1992-95 war. They said they would try to dispute it by all legal means.

"We expect, regardless of a veto expected to be filed by Bosniaks, to complete the procedure by end of August and kick-start preparations for the referendum on Sept. 25," said Bosnian Serb President Milorad Dodik. "The decision will be enforced as a binding law".

Dodik had also threatened to push for referendums on the authority of the national court which he says is biased against Bosnian Serbs.

Statehood Day celebrations were held this year, also in defiance of the top court's ruling.

(Reporting by Gordana Katana, writing by Maja Zuvela; Editing by Daria Sito-Sucic and Andrew Heavens)