There has been fighting in the southern Philippines for decades
The Philippines' Supreme Court has blocked the signing of a territorial deal between the government and Muslim separatists in the south of the nation.
The two sides were due to formalise a deal on Tuesday widening an existing Muslim autonomous region.
But the court acted after Catholic lawmakers from the region said they had not been adequately consulted.
Rebels from the Moro Islamic Liberation Front have been fighting for self-rule in the southern Philippines for years.
The agreement was intended to kick-start peace talks aimed at ending nearly four decades of conflict.
In late June, the two sides agreed that a six-province Muslim autonomous region on the island of Mindanao would be expanded to include 712 villages, subject to the agreement of villagers in a vote.
But some Catholic lawmakers fear such a move could lead to more sectarian violence, and there have been large rallies against the agreement.
"Do not build a Berlin Wall among the people in Mindanao," Celso Lobregat, mayor of the mainly Catholic city of Zamboanga, told a 10,000-strong crowd.
The court has now temporarily suspended the deal and ordered the government to make its case at a hearing on 15 August.
The deal remains controversial.
Supporters, including President Gloria Arroyo, say it is badly needed to help end a war that has cost more than 100,000 lives and left the southern Philippines mired in poverty, says the BBC's Michael Barker in Manila.
But critics say the agreement is tantamount to allowing the establishment of an independent state within the country, something that is outlawed by the constitution, our correspondent adds.