The poster was installed at a commuter rail station in the heart of the Spanish capital ahead of regional elections in Madrid on May 4.
It features an old white-haired old woman with a sad expression beside a dark-skinned youth wearing a hooded top, his mouth covered with a bandana.
The poster claims that an unaccompanied migrant minor receives €4,700 ($5,650) per month, “your grandmother a pension of €426 per month”.
Vox, the third-largest party in Spain’s parliament, has made attacks against the arrival of unaccompanied migrant children to Spain one of the main themes of its campaign.
At rallies its leaders regularly link them to rapes and other crimes.
After receiving several complaints, including one from Prime Minister Pedro Sanchez’s leftist government, the Madrid public prosecutor’s office has opened a probe into a possible hate crime.
Minister for Social Rights Ione Belarra said the poster was “an attack against the most vulnerable children of our country.”
“Democratic institutions can’t remain indifferent when confronted with this discrimation against youth and the spread of hate by the far right,” she tweeted.
Las instituciones democráticas no pueden permanecer indiferentes ante la discriminación a la infancia y la extensión del odio de la ultraderecha.
— Ione Belarra (@ionebelarra) April 20, 2021
The poster was also criticised by Catholic charity Caritas, Save the Children and Unicef, who asked in a joint statement that Madrid’s electoral board investigate if it broke the law.
In the face of this row, left-wing parties have stepped up pressure on the outgoing head of Madrid’s regional government, Isabel Diaz Ayuso, to ensure she does not enter into a pact with Vox if needed to stay on in power after the polls.
Ayuso’s conservative Popular Party lacked a majority in the outgoing Madrid regional parliament and relied on the backing of Vox to govern.
She has not ruled out a new pact with Vox but sought to distance herself from the controversy, saying Wednesday that the poster was “not true” and was “out of place”.