Spain is preparing to legalise around half a million undocumented migrants, causing immigration officers across the country to call for a strike.

Unions say civil servants are unprepared to 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.