Sustainable public procurement (SPP) has been incorporating sustainability issues into commercial relationships between governments and businesses. SPP implementation can be undertaken with the support of organisational changes. Research has highlighted the role of stakeholders' collaboration in driving changes for SPP. However, there is limited understanding of the forces fostering and blocking the implementation of collaboration. This paper analyses drivers for and barriers to implementing collaboration in SPP, based on semi-structured interviews with SPP practitioners from different world regions (Brazil and Sweden). This paper provides a catalogue of forces influencing organisational changes towards collaboration in SPP (categorised into internal, connecting-protagonist, connecting-supporting and external), including 23 drivers, 40 barriers and 24 strategies to overcome such barriers. This paper also proposes an understanding of how organisational changes for collaboration are interconnected with changes for implementing SPP. Organisational changes for SPP must be understood as multiple and composite instead of single and monolithic.