The issue of possible independence for Catalonia was the number one political problem facing Spain, the leader of the socialist party Pedro Sánchez said in an article published by centre-left daily El País.
Catalonia's planned November 9th vote on the issue of whether the region should split from Spain "changes the very idea of what Spain is," Sánchez wrote.
Spain's Madrid-based central government has repeatedly said the controversial vote in the region with 7.5 million people is illegal.
It now plans to launch an appeal in the country's Constitutional Court after the Catalan government recently passed a regional law green-lighting the staging of the poll.
In a strongly-worded analysis, the opposition leader accused the Catalan government of using "word games" in a bid to add a veneer of legality to the vote, when in fact it was trying to "override the law".
Sánchez slammed Catalan leaders for describing the non-binding vote as a "consulta" (consultation) and not a referendum, and for making a plea for Catalans "right to decide" rather than talking about self-determination.
The newly-elected leader of the struggling Socialists — many in Spain blame the previous PSOE government for the country's economic crisis — went on to support the position of Spain's conservative ruling Popular Party which argues the vote is illegal.
But the fresh-faced 42-year-old opposition leader said the government led by Spanish Prime Minister Mariano Rajoy now needed to show it could "work for the future" and not just for the status quo.
Regardless of whether the Catalan vote goes ahead or not on November 9th, Spain needs to deal with underlying issues, Sánchez wrote.
"Our collective duty is to recognize and deal with (the problems) so that they don't damage the current system of coexistence and welfare outlined by the Spanish constitution," he added.
Sánchez called on the Spanish government to overhaul the constitution, giving clear powers to the country's regions, and ensuring they have adequate and sufficient funding.
The "singularity" of certain regions also needs to be recognized, he said.
"We need a new boost for our democracy, one that deepens and extends it, which promotes citizen participation, revives the Parliament, improves the representation of elected politicians, reduces immunity and assures the independence of our justice system," Sánchez said, outlining his reasons as to why Spain needs constitutional reform.
The opposition leader also said Spain's constitution needed updating to include the possible consequences of membership of the European Union.
Government spokesperson Alfonso Alonso repsonded to Sánchez's suggestions by saying they comprised a "tactical" move, with the Socialist trying to position themselves as a third alternative in opposition to the government and the Catalan nationalists.
He said the PSOE leaders ideas were the "old proposal" of former opposition chief Alfredo Pérez Rubalcaba, "without greater definition".
The Secretary-General for the ruling Popular Party Dolores de Cospedal, meanwhile, said the Socialists were not suggesting constitutional reform but a whole new referendum.