BRUSSELS — The European Commission’s decision to send a commissioner to a meeting of Donald Trump’s Board of Peace was “a grave error of judgement,” the European Parliament’s social democratic, liberal and green groups said in a letter sent Friday to Commission President Ursula von der Leyen, seen by POLITICO.

Dubravka Šuica, the European commissioner for the Mediterranean, was in Washington on Thursday for the first formal meeting of Trump’s Board of Peace, which is designed to oversee Gaza’s demilitarization and reconstruction under a U.N.-endorsed ceasefire framework.

The EU has stated it does not endorse the initiative and it will not participate as a member, but the decision for Šuica to attend caused outcry.

“Participation by a Commissioner in a politically contested diplomatic initiative without a defined mandate disrupts the institutional balance and risks breaching the principle of sincere cooperation between EU institutions and Member States,” the letter says. “Respect for the Treaties is essential.”

The leaders of the three parliamentary groups said the Commission did not have a mandate to make such a decision. They add that the attendance of the Commission gave visibility and legitimacy to the Board of Peace while weakening the authority of the United Nations.

“We command the President of the European Commission to clearly disengage from Donald J. Trump’s Board of Peace, to refrain from sending any observer to its meetings, and to cease any form of participation,” they add.

The European People’s Party — von der Leyen’s political family — did not sign the letter. An EPP spokesperson did not reply to a request for comment by the time of publication.

Bulgaria and Hungary are the only EU members to have formally signed up to the peace board, with Italy and Romania attending as observers.

The Commission on Friday defended the decision. “At least half of the member states, representative of the member states, participated in this meeting,” said Commission spokesperson Guillaume Mercier. “We are not becoming a member of the Board of Peace,” he added, insisting Šuica attended to participate “in the wider discussion to ensure a safe future for Gaza and for the Palestinians.”

“It is within the remit of the Commission as an external representative of the Union to accept invitations of this kind, as a matter of international courtesy,” chief spokesperson Paula Pinho said when asked about the legal basis for the Commission’s participation in the meeting.

The political groups’ letter follows outcry from France. The Commission “should never have attended,” French Foreign Minister Jean-Noël Barrot posted on X late Thursday. “Beyond the legitimate political questions raised by the ‘Board of Peace,’ the Commission must scrupulously respect European law and the institutional balance in all circumstances.”

Gerardo Fortuna and Nicholas Vinocur contributed to this report.