Montenegrins have failed to elect a president because the turnout fell below the 50 per cent legal minimum.
The Centre for Monitoring Elections (CEMI) said 45.7 per cent of the electorate had cast votes when the polls closed in the tiny Yugoslav republic of 600,000 people.
The law requires turnout of at least 50 per cent for the ballot to be valid. Observers had predicted the turnout would fall below the threshold due to political disillusionment made worse by a crime scandal and an opposition call for a boycott.
CEMI saidb Mr Filip Vujanovic, the main ruling party candidate and the frontrunner according to pre-election opinion polls, won support of 83.9 per cent of those who voted, while the remaining votes were shared among 10 other candidates.
Montenegro's election law requires a repeat vote with the same candidates. It will be held next month. If that fails, the whole election process must begin again.
Serbia and Montenegro - once the closest of allies in the old, larger Yugoslavia - saw relations worsen after divisions over the Kosovo crisis. They are due to transform Yugoslavia into a looser federation next month under a European Union-backed deal.
Serbia has also failed to elect a president as low turnout has also rendered the vote invalid. Serbia has tried three times this year to elect a president.