Prime Minister Mariano Rajoy's conservative Popular Party would win an election with 28.2 per cent if the vote were it held today, up from 25.6 per cent in April, the survey by the state run CIS research centre showed.
The Socialist Party would come in second with 24.9 per cent support, compared to 24.3 per cent during the last CIS survey published in April.
New anti-austerity party Podemos, a close ally of Greece's ruling Syriza, came in third place with 15.7 percent support, compared to 16.5 per cent in April.
Both Podemos and the Socialists have sought to portray themselves as the only viable opposition to the Popular Party, which has ruled Spain since 2011.
Also Read
The centrist Ciudadanos party also saw its support drop to 11.1 per cent from 13.8 per cent in April.
No clear majority would emerge from the vote, forcing the main parties to try to form a coalition, according to the poll, the nation's most closely-watched political survey.
Spain's unemployment rate of 22.4 per cent's the second highest in the European Union after Greece's.
The poll was based on 2,486 interviews made across Spain between July 1 and 9.