It is not clear if the UN report, which is expected to be given to diplomats involved in the Kosovo status negotiations, will be immediately made public.
Bozidar Djelic, considered most likely to become the prime minister in a coalition government of pro-Western reformist parties, warned that releasing the UN report before the formation of a new government would only serve the ultra-nationalists.
"The so-called Democrats cannot agree on anything, let alone the new government"
Tomislav Nikolic, leader of the Radical party
The ultra-nationalist Radicals won 28.3 per cent of Sunday's vote, the state electoral commission said Monday.
But that was not enough for the party to govern alone, leading to a race by the pro-democracy camp to unite and form a government.
Tomislav Nikolic, the Radical leader, said Monday that Tadic should offer his "strongest party" a mandate to form a government.
"But I know he won't do it," Nikolic said.
He predicted new elections by the end of the year because "the so-called Democrats cannot agree on anything, let alone the new government".
Retention of office
Vojislav Kostunica, the current prime minister, whose centre-right Popular Coalition came in third in the polls, insists on retaining the influential post.
Tadic's Democrats, who were second after the Radicals, want the post for themselves.
"The Democratic Party will not give up the right to have its candidate for prime minister as we got the most seats"
Jelena Markovic, Democrat spokeswoman
Tadic's Democratic Party gained 22.6 per cent, and the Popular Coalition had 16.3 percent, according to official results with about 70 per cent of ballots counted.
"I don't think it will be easy to form a government in these circumstances," said Jelena Markovic, a Democrat spokeswoman.
"The Democratic Party will not give up the right to have its candidate for prime minister as we got the most seats."
However, the ultra-nationalists are also unwilling to make concessions to other parties, according to Slobodan Antonic, an analyst at Belgrade University.
He said: "The Radical party itself refuses to make coalitions… They think other parties are too corrupt.
"They are the far-right party and it is difficult for them to find a coalition partner more right than they are."