Brussels fears Spain’s mass amnesty for illegal migrants may prompt an E.U. member state to shutter its borders, European Commissioner for Asylum and Migration Magnus Brunner declared Wednesday.

Brunner delivered the his remarks at “Wake Up, Spain! Wake Up, Europe! ‘Growth, Cohesion, and Uncertainty,'” an ongoing symposium hosted by the digital newspaper El Español and others in Madrid between April 13 – 17.

The commissioner addressed the fragility of the Schengen free movement zone and pointed out that ineffective management of irregular migration erodes trust among Member States, which could trigger the reintroduction of internal border controls and “jeopardize the very essence of European freedom of movement.”

“If irregular immigration is not properly managed, internal borders will be closed,” Brunner said, per El Español.

The commissioner delivered his warning hours after the government of Spanish socialist Prime Minister Pedro Sánchez officially launched its highly controversial plan to grant mass amnesty to half a million illegal migrants living in Spain — something that Brunner reportedly warned last week may have “consequences’ for the European Union.

Brunner argued during his online participation at a symposium in Madrid that “security cannot be separated from the Schengen Area” and that free movement, which he reportedly described as the “heart of European freedom,” is only “possible because we maintain robust controls at our external borders,” stressing that it is a “responsibility shared by all Member States.”

“In this regard, he [Brunner] noted that what happens at border crossings in a country like Spain inevitably affects the entire Union, urging members to maintain cohesion to prevent the Schengen system from collapsing due to a lack of coordination,” El Español wrote.

Brunner also reportedly defended the need to strengthen cooperation with countries of origin and transit — citing Mauritania and Senegal — to curb the outflow of illegal migrants at the source.

Last week, responding to a request presented at the European Parliament by lawmakers from the Spanish center-right People’s Party (PP), Brunner reportedly explained through an internal communication that Brussels had not received any official note from the Spanish goverment over its mass amnesty plans.

El Español recounted that EU lawmakers from Germany, Austria, Italy, Poland, Sweden, Portugal, and Hungary widely critizized Spain’s plans during a February 10 parliamentary debate — warning that the mass amnesty plans “undermines our joint efforts to strengthen our borders and combat illegal migration.”

At the time,  Brunner warned the Sánchez’s government that obtaining a residence permit in the European Union “is not a blank check,” stressing that “each Member State must ensure that its decisions do not have negative consequences for other parts of the EU.”