The onset and evolution of magnetic fields in laboratory and astrophysical plasmas is determined by several mechanisms1, including instabilities2, 3, dynamo effects4, 5 and ultrahigh-energy particle flows through gas, plasma6, 7 and interstellar media8, 9. These processes are relevant over a wide range of conditions, from cosmic ray acceleration and gamma ray bursts to nuclear fusion in stars. The disparate temporal and spatial scales where each process operates can be reconciled by scaling parameters that enable one to emulate astrophysical conditions in the laboratory. Here we unveil a new mechanism by which the flow of ultra-energetic particles in a laser-wakefield accelerator strongly magnetizes the boundary between plasma and non-ionized gas. We demonstrate, from time-resolved large-scale magnetic-field measurements and full-scale particle-in-cell simulations, the generation of strong magnetic fields up to 10–100 tesla (corresponding to nT in astrophysical conditions). These results open new paths for the exploration and modelling of ultrahigh-energy particle-driven magnetic-field generation in the laboratory.