Spain is preparing to legalise about 500,000 undocumented migrants, causing immigration officers across the country to call for a strike.
Unions say civil servants cannot handle such large numbers.
Spanish Prime Minister Pedro Sanchez has described it as an “act of justice and a necessity”.
“Spain is ageing … without more people working and contributing to the economy, our prosperity slows and our public services suffer,” he said on X.
The amnesty is a central plank of Mr Sanchez's progressive agenda to harness the economic benefits of migration for Spain's ageing population, even as other European governments move to tighten their borders to head off political challenges by the far-right.
Online applications will open on Thursday, after the government rubber-stamped the initiative at a cabinet meeting this week.
However, immigration officers warn the system remains unprepared for the challenge and have threatened to strike from April 21, a day after in-person appointments open, halting all immigration applications in protest at the lack of resources allocated to the process.
“The government is once again implementing a new regularisation without giving offices enough economic resources to handle it,” Cesar Perez, a union leader for Spain’s immigration officers, told Reuters.
In a bid to alleviate pressure on an already overburdened immigration system, only five of the country's 54 immigration offices will be responsible for handling applications, with the rest distributed among social security offices, post offices and NGOs, Spanish union CCOO said.
Spain's near 50 million population has swelled in recent years to include around 10 million born abroad. Spanish think tank Funcas estimates 840,000 undocumented migrants make up part of the workforce at present.
The centre-right opposition Popular Party has deemed the drive reckless, despite previous conservative governments pushing through similar measures.
Isabel Diaz Ayuso, president of the community of Madrid and a prominent figure in the party, has threatened to launch an appeal in court.
