Distant metastasis is the most lethal hallmark of cancer. Nevertheless, due to a lack of suitable markers, the emerging motility of cancer cells only has a negligible impact on current diagnosis and prognosis. In cancer clusters of solid tumours, cooperative unjamming transitions generate regions of unjammed motile cells, as we could show by vital cancer cell tracking in patient-derived tumour explants. Thereby, cancer cells and their nuclei collectively deform/elongate to squeeze past each other. The cells' differing shapes and densities, and nuclei of jammed and unjammed cancer cells can serve as the first reliable motility marker in static images such as histopathological slides. A retrospective clinical study that strongly correlates unjamming with distant metastasis demonstrates that unjamming within primary tumours is part of the metastatic cascade. With the histological slides of two independent breast cancer patient collectives, we train (N=688) and validate (N=692) our quantitative prognostic index based on unjamming regarding metastatic risk. Our index corrects for false high- and low-risk predictions based on nodal status. Combining information derived from the nodal status with unjamming may reduce over- and under-treatment and significantly improve the understanding of the metastatic cascade.