Spain's government has approved plans to give legal status to 500,000 undocumented migrants, allowing them to be integrated formally into the workforce.

Prime Minister Pedro Sánchez described his government's decision as both "an act of justice" and a necessity for Spain.

In a letter to Spaniards posted on social media, Sánchez, a socialist, said the mass legalisation sought "to acknowledge the reality of nearly half a million people who already form part of our everyday lives".

Spain's conservative opposition People's Party (PP) has pledged to attempt to block the legalisation, which it said rewards illegal migrants and would encourage more to come.

The government's plan will offer a one-year, renewable residence permit to undocumented migrants. In order to be eligible, applicants must prove that they have already spent five months living in Spain and have a clean criminal record. They have between 16 April and the end of June to apply.

Sánchez said migrants helped "build the rich, open and diverse Spain that we are and to which we aspire".

The prime minister said these migrants are needed to sustain the economy and public services in a country whose population is ageing. He also said it was the right course of action for a nation which in the past had seen many of its own nationals emigrate in search of better opportunities.

The Funcas think-tank estimates that there are around 840,000 undocumented migrants in Spain, the vast majority of whom are Latin American.

"This is going to benefit a lot of people, give them access to work and to a better quality of life," said Ricardo, a Bolivian graphic designer who has been unable to maintain stable employment and plans to apply for the scheme. "And it means more money for the Spanish state and more workers will be legally available for employers."

The opposition has said that the government's estimates are wrong and that about one million migrants could apply for the scheme, with PP describing the plan as an "outrage".

The Catholic Church, by contrast, has supported the government's legislation.

The scheme comes as many of Spain's European neighbours tighten controls on immigration.

Both Socialist and PP governments have implemented migrant amnesties in the past. The most recent one was in 2005, when 577,000 people received residency under a Socialist administration.