Nigeria's senate has approved a motion declaring that last year's handover of the oil-rich Bakassi peninsula to Cameroon was illegal.
The handover is unpopular with Nigerian residents
The upper house of parliament said that no part of Nigeria could be ceded without changing the constitution.
Senators called on the government to halt the transfer of areas along the border further north.
In 2002, the International Court of Justice ruled in favour of Cameroon in the two countries' border dispute.
The senate also called for more aid for those displaced by the handover - but did not say that Nigeria should retake Bakassi.
"The agreement reached to cede the Bakassi region to Cameroon was not tabled before the National Assembly," Senator Muhammad Mana told the BBC.
"Therefore everything on the agreement should be stopped until it has been ratified by the National Assembly.''
The vote comes at a time of heightened tensions in the area.
Nigeria reinforced troops on its side of the border after 21 Cameroon troops were killed in Bakassi two weeks ago.
It is not clear who carried out the attack - Nigeria's oil militants have denied claims they were responsible.
Nigerian troops withdrew in August 2006 but the peninsula will remain under Nigerian civil administration until 2008.
Most of the residents were Nigerian and bitterly opposed the handover.
Nigeria and Cameroon sought arbitration after a series of bloody clashes in the 1990s.
Bakassi juts into the Gulf of Guinea, an area which may contain up to 10% of the world's oil and gas reserves. It is also rich in fish.
The peninsula has been administered by Nigeria since independence from Britain in 1960.
However, Cameroon based its claim of sovereignty over the region on maps dating back to the colonial era.