The Spanish Socialist Party (PSOE) of Prime Minister Pedro Sanchez would win next Sunday's general election but fall well short of an overall majority and would need the support of more than one party to form a government, according to an opinion poll published on Sunday.

The poll, compiled by investigative company 40dB for the prestigious 'El Pais' newspaper a week before the April 28 election, predicted that the PSOE would be the most voted for party winning 28.8 percent of the votes and 129 seats in the 350-seat Congress.

The right wing People's Party (PP) would be the second most popular party with 17.8 percent of the votes and 75 seats, while the center right Ciudadanos would claim 14.1 percent of the votes and 49 seats, the poll indicated.

The left wing Unidos Podemos was predicted to win 13.2 percent of votes and 33 seats, while the extreme right wing Vox would win 12.5 percent of the votes and 32 seats, allowing the party led by former PP member Santiago Abascal to enter Congress for the first time.

Meanwhile other parties, including Basque and Catalan nationalist parties and the animal rights party PACMA, would take 32 seats and 11.3 percent of the votes.

If the findings were correct then a coalition between the PSOE and Unidos Podemos would have 162 seats in Congress - 14 short of the 176 needed for a majority, leaving Sanchez needing the support of nationalist parties in order to govern.

He would be able to form a coalition government with Ciudadanos, but Ciudadanos leader Albert Rivera has already said he is not willing to help Sanchez govern after the PSOE held talks with Catalan nationalists in a failed attempt to pass his government's 2019 budget.

The poll also highlighted the fractured nature of the right wing vote in Spain, with the rise of Vox - a party with an anti-immigration, anti-Muslim and anti-feminist ideology - taking support away from the PP and Ciudadanos.

However, 26 percent of the 2,000 people polled said they had still not decided who they would vote for. That means the final week of the campaign, which includes two televised debates between Sanchez, Rivera, Pablo Iglesias (Podemos) and Pablo Casado (PP), will be decisive.