The EU also called on Belgrade's reform-led parties to use the period until May 15 under Serbia's constitution to form a "democratic, majority-based government" which reaffirms Serbia pro-European policy.
The EU call was mirrored hours later by Nato, which urged Belgrade in a statement to come up with a government "that would continue Serbia's path to Euro-Atlantic integration".
Nikolic's successful campaign to become speaker was backed by both Vojislav Kostunica, the outgoing prime minister, a moderate nationalist and leader of the Democratic Party of Serbia, and Milosevic's weakened Socialists.
The Radical Party opposes the handing over Ratko Mladic, a fugitive general, to the war crimes court - this is a key EU demand blocking Serbia's membership hopes.
These parties are cool to the EU and NATO, and suspicious of economic liberalism and market reforms.
A January 21 election produced a hung parliament and the period since has seen fruitless coalition talks between Kostunica and the democrats of Boris Tadic, the country's pro-western president.
If there is no government by May 14, new elections must be called.
The campaign could coincide with the loss of Serbia's Kosovo province, whose Albanian majority expects to win independence by the summer with western backing.
The EU, wary of a nationalist backlash on Kosovo and seeing a Serbia on course for EU membership as vital to Balkan stability, has urged Kostunica and Tadic to unite.