Shallow, mixed-phase clouds within marine cold air outbreaks (CAOs) frequently form over the North Atlantic. their shortwave radiative effect is modulated as stratocumulus decks break up into cumulus clouds to the south. Microphysical processes controlling their phase remain poorly represented by climate models; of these, secondary ice production (SIP), describing mechanisms producing new ice crystals from existing primary ice, is a major contributor to uncertainties in the mixed-phase cloud response to future warming. We examine in-situ measurements of cloud microphysical properties made using the UK FAAM BAe-146 research aircraft within CAOs over the Labrador Sea as part of the October–November 2022 M-Phase field campaign. Measured ice particle concentrations frequently exceeded ice-nucleating particle (INP) concentrations at all in-cloud temperatures, highlighting the importance of SIP in these clouds. Peak ice concentrations were observed within the Hallett-Mossop (H-M) process temperature range (-3 to -8 °C), four orders of magnitude above expected INP concentrations. SIP regions contained large, rimed columns and graupel mixed with smaller columnar crystals (<200 µm), indicative of the H-M process. Splinter production rate calculations indicated the H-M process could account for most ice production in the largest ice enhancement regions. A secondary zone of SIP activity, between -15 and -18 °C, comprised fragile, branched crystals, aggregates and ice fragments, consistent with laboratory studies of ice-ice collisional breakup. SIP amplified across the stratiform-to-convective regime transition, favouring weak-to-moderate updrafts (0 to +2 m s^-1) containing high concentrations of large liquid droplets, suggesting regime-aware SIP schemes would benefit future CAO modelling.