Bougainville MPs have rejected a push by the government to allow presidents to sit for three terms.
If successful the legislation would have allowed the incumbent for the past 10 years, John Momis, to stand in the elections in May.
Some commentators in Bougainville have suggested Mr Momis, who has been involved in the province's political affairs for 50 years, is needed to see through the negotiations on the results of the referendum on independence from Papua New Guinea.
But MPs denied the government the two-thirds majority needed to change the constitution.
On a second matter, MPs agreed to continue with the three seats set aside for ex-combatants, which were due to be abolished at the coming election.
In December, an overwhelming majority of Bougainvilleans voted for independence, but the result needs to be ratified by PNG's Parliament to come into effect.