MADRID (Reuters) - Spain’s Socialists increased their lead in a poll published on Monday in newspaper ABC that gave them 31.5 percent of votes, but fell short of a majority ahead of a general election on April 28.

FILE PHOTO - A man walks past an electoral poster of Spain's Socialist (PSOE) leader and current Prime Minister Pedro Sanchez outside the PSOE headquarters in Madrid, Spain, April 12, 2019. REUTERS/Susana Vera

It is the third poll in two days that shows the Socialists expanding their advantage, and forecasting that a coalition of three right-wing parties - People’s Party (PP), Ciudadanos and far-right Vox - would not get enough seats to form a parliamentary majority.

Socialist Pedro Sanchez could be reelected as prime minister if he forms a coalition with the support of at least two of the array of parties - far-left Podemos and a Catalan pro-independence group - that backed him last June when he won a vote of confidence against PP’s government at the time, according to the ABC poll.

But it also showed that 33 percent of voters have not decided who they will vote for next Sunday.

The Socialists would have between 134 and 139 seats in the 350-seat parliament, gaining up to 54 new seats from the previous election in 2016.

Sanchez’s support increased by 0.2 percent from a previous poll published by the same newspaper on April 16, and by five points from a poll last January.

The second most voted party would be PP, with 20.1 percent of support, 0.8 percent less than in the previous week poll. The main conservative party would have between 81 and 86 seats in parliament, losing up to 56 from the last election.

Ciudadanos would receive 13.9 percent of votes, equivalent to 42 to 44 seats, above the 32 that they currently have.

Newcomer Vox would be the fourth party in seats (30 to 32) but, with 11.4 percent of votes, would be below anti-austerity Podemos, which would receive 12.1% of support but that would translate in a smaller amount of parliamentary seats (27).

Vox’s support fell by 0.4 percent from the April 16 poll, but it would still be the first time in almost four decades that an extreme-right party enters the Spanish parliament.

A coalition of PP, Ciudadanos and Vox would get 45.4 percent of votes, equivalent to between 153 and 162 seats, but this would also be short of the 176 seats needed to secure a parliamentary majority.

The poll was conducted by GAD3 with 10,000 interviews between April 1 and 19. The margin of error was of 1 percent.