A poll forecasts that Spain's conservative Popular Party could boost its share of the vote in fresh elections next month but not by a sufficient margin to break the stalemate that has stopped politicians from forming a government since December.

The survey by private pollster Metroscopia published Sunday in newspaper El Pais also suggests the newcomer Podemos Unidos alliance would oust the Socialists as the main opposition party.

December's inconclusive poll led to new elections being called for June 26.

Metroscopia predicts support for the Popular Party will increase to 29.9 percent from 20.8 percent in December, with the Socialists dipping to 20.2 percent from 21.9 percent.

The pollster says it quizzed 1,200 adults in a May 17-18 survey that has a margin of error of 2.9 percentage points.