In accordance with the Article 24 (2) of the Horizon 2020 Rules for Participation (RfP) and the Article 41 (3) of the Grant Agreement for Horizon 2020 (GA), the participants have to conclude a consortium agreement (CA) if the work programme does not rule otherwise.
However, neither the RfP nor the GA stipulates that a CA has to be concluded before the GA is signed. The Annotated Model GA only strongly recommends ("should in principle") that the CA be negotiated and concluded before the action starts because not having a CA at this moment could delay and jeopardise the action.
The situation would change if the work programme exceptionally stipulates that a CA be concluded before the signature of the GA. Such additional participation conditions might be included in accordance with the Article 9(5) of the RfP. In such case, if the European Commission becomes aware of the absence of the conclusion of the CA it would not sign the grant agreement.