The UN administrator in Kosovo, Michael Steiner, within minutes declared "null and void" the resolution adopted by the ethnic Albanian-dominated assembly which rejected a border agreement between Yugoslavia and Macedonia.
Kosovo, which borders Macedonia, legally remains part of Yugoslavia. The unanimously passed resolution is being viewed as an attempt by the province to act as an independent state.
The incident is also seen as the most serious rift in relations between the province's ethnic Albanian leadership and the UN since the world body took charge of the province in June 1999.
The vote is also likely to earn strong criticism from Belgrade - and has already prompted a walkout from the assembly by the Serb deputies.
Warnings
Ethnic Albanian MPs - who press for Kosovo's outright independence - have been unhappy with the border agreement between Yugoslavia and Macedonia since it was signed last year.
Assembly powers
Economic and financial policy
Fiscal and budgetary issues
Education
Health
Social welfare
Organisation of judicial system
Transport and communications
Sport and culture
Agriculture
Tourism
They say that over 4,000 hectares of land were removed from Kosovo without any consultation with the local population.
The BBC's Nicholas Wood in Pristina says that while some UN officials have sympathised with the MP's grievances, the UN Security Council and the European Union, have warned that the assembly has no rights to discuss issues affecting the region's borders or internal security.
Letters from the Security Council and the EU not to go ahead with the vote were presented to the assembly before the session.
They warned that Kosovo's reputation would be damaged if the motion was adopted.
Walkout
Nevertheless, the assembly went ahead with the resolution.
This in turn prompted the Serbian deputies to leave the parliament, saying they would no longer take part in the assembly or the province's government unless the motion was rejected.
The 120-strong Kosovo's first multi-ethnic assembly opened its inaugural session in December 2001, following elections the month before.
But the UN retained the final say on the most contentious policy issues, banning the assembly to vote for the province's independence.