MADRID (Reuters) - Spain's third national election in a year would deliver yet another hung parliament and give rival parties no new options in terms of forming a government, a poll published on Sunday showed as the prospect of another ballot edges closer.

Leaders have struggled to form a new administration since last December after upstart political parties won widespread support in a general election - forcing several forces, including some traditional rivals, to try and reach pacts.

After the Dec. 20 ballot, a fresh election on June 26 did nothing to resolve the impasse, with all parties, including the conservative People's Party (PP) of acting Prime Minister Mariano Rajoy, falling short of a parliament majority.

Rajoy lost a confidence vote among lawmakers in early September on his bid for a second term, setting the clock ticking on yet another election that would likely fall in December.

The PP would win once again, an opinion poll by GAD3 published in ABC newspaper showed, increasing its parliamentary seats to 142 from 137 in June and 123 on Dec. 20.

But that would still be shy of the 176 needed for an absolute majority, even if the PP managed to again draw on support from smaller liberal party Ciudadanos ("Citizens") in any subsequent confidence vote.

Ciudadanos' seats would fall from 32 in June to 30 in another election, according to the poll, drawn from 802 interviews in the week after Rajoy's failed attempt to form a government.

Other parties would also lose ground, including the center-left Socialists, who have steadfastly opposed another PP term in office.

The Socialists would garner 81 seats, compared to 85 at the last election, while Unidos Podemos ("Together We Can"), a leftist alliance that erupted onto Spain's political scene in the wake of a deep economic crisis, would get 70, down from 71 seats last time.

Spaniards' frustration with the eight-month political deadlock is on the rise, meanwhile, according to a survey published in El Pais. It said 71 percent of people were against a third election, and that abstention would rise sharply, to 37 percent from 30.2 percent in June.

Spain's economy has so far largely withstood the uncertainty, removing some of the urgency leaders might otherwise have faced to form a government.

(Reporting by Sarah White; Editing by Stephen Powell)