The first results from a weekend referendum showed on Monday that Serbians voted by only a tiny margin to support a new Constitution, despite government attempts to rouse support by making Kosovo a central theme.
According to preliminary official results announced by electoral authorities, the new charter was endorsed by a majority of only around 100,000 voters from the 6.6 million-strong electorate.
This was in spite of an intense, month-long government campaign that appealed to voters' attachment to the breakaway ethnic-Albanian majority province of Kosovo, which nationalists consider to be the country's heartland.
In the two-day referendum, 52.3 percent of voters backed the Constitution, the Electoral Commission said.
Under Serbian law, at least half the electorate had to support the charter for its approval.
The commission said turnout was 54.2 percent despite a rare show of unity by all major political parties and the backing of the Serbian Orthodox Church.
The referendum on the charter -- whose preamble stresses Kosovo is an "integral" part of Serbia -- was held over two days to give Serbia's notoriously apathetic voters as much time as possible to cast ballots.
A relieved-looking Serbian Prime Minister Vojislav Kostunica hailed the success of the referendum when he emerged to make an address on state-run television late on Sunday.
"This is the moment in which Serbia clearly approves its unity, [supporting the claim] that Kosovo is an integral part of Serbia," Kostunica said.
The new charter is also the first to replace the one adopted under the rule of former president Slobodan Milosevic, which introduced a multi-party political system in 1990.
The referendum results, which are due to be officially confirmed tomorrow, meant the moderate nationalist prime minister's fractious coalition government narrowly avoided a political crisis.
A failure of the vote would have been a major blow to Kostunica's government, already under pressure over the likely loss of Kosovo and the suspension of its EU integration talks because of its failure to arrest war crimes fugitives.
In Brussels, the European Commission welcomed Serbia's backing of the new Constitution, while in Strasbourg the Council of Europe hailed the "peaceful, and orderly conduct of the vote."
However, questions remained about the validity of the vote, particularly over a sudden upsurge in voting in the referendum's closing hours after an opposition party charged the vote "was marred by a series of irregularities."
Four minor opposition parties said in a statement that they had filed "several hundred complaints due to forging [of] referendum results."
The new charter was unanimously backed by the Serbian parliament a month ago in a rare show of political unity between the conservative government and liberal and ultra-nationalist opposition parties.