MEXICO CITY: Mexico's ruling party and its allies in the lower house of Congress extended their slim majority in last weekend's elections, according to a preliminary estimate from the National Electoral Institute (INE).

President Enrique Pena Nieto's Institutional Revolutionary Party (PRI), the Green Party and the smaller New Alliance Party (PANAL) went into the vote with a narrow majority of 251 seats in the 500-member lower house.

The internal INE estimate seen by Reuters on Saturday showed the three parties winning 260 seats. This was largely because they performed significantly better than the opposition in the battle for the 300 seats awarded by relative majority, where the party with most votes in a constituency wins.

The other 200 lower house seats are divided up among the parties by proportional representation of the overall voting.

Final INE lower house results are due on Sunday. An INE official, speaking on condition of anonymity, said the preliminary estimate would likely be confirmed by the institute.

The lower house result could still be affected by recounts or appeals to the electoral tribunal, and if the Labor Party (PT), a small left-leaning group, ended up clearing the three percent vote hurdle needed to enter Congress.

An initial INE vote count showed the PT falling just short of that threshold, which meant any seats it would have won through proportional representation are redistributed.

The same tally showed the PRI and its allies captured almost 40 percent of the total vote last Sunday, down about two percentage points from the 2012 elections.

The PRI's closest rival, the center-right National Action Party (PAN), saw its share of support drop some five points to barely 21 percent, with the vote fractured by a bigger selection of parties and independent candidates.

PAN won 108 lower house seats, the INE estimate showed, with the main leftist opposition group, the Party of the Democratic Revolution (PRD) with 56 seats, barely half of its prior total.

The PRD lost support due to the emergence of Morena, a new leftist party formed by two-time presidential runner-up Andres Manuel Lopez Obrador, which was projected to win 35 seats.

The PRI and its allies are a few seats short of a majority in the Senate, which was not up for re-election.

Pena Nieto is not expected to rely on Congress as much in his last three years, having fulfilled the bulk of his main legislative pledges, including measures to end the state oil and gas monopoly and open up the telecoms sector to competition.