The main autochthonous Portuguese sheep breeds exploited for meat production are Churra-Galega-Bragançana (CGB) and Bordaleira-de-Entre-Douro-e-Minho (BEDM), whose quality must be optimised in order to ensure adequate income levels for sheep producers. The study aimed to elucidate how intrinsic properties affected the evolution of spoilage microorganisms in refrigerated vacuum-packed (VP) lamb meat from BEDM and CGB breeds. Thirty CGB and 30 BEDM lambs were four months old when slaughtered. Physicochemical analyses (i.e., pH, water activity and proximate composition) were performed on meat on day 1 after slaughter, while microbial counts (mesophiles, psychrotrophic, lactic acid bacteria and Pseudomonas spp.) were essayed on VP meat on days 3, 9 and 15. Mixed-effects models were adjusted to each of the microbial groups to assess the effect of breed, time of maturation and each of the intrinsic properties of meat, in separate. Breed had a significant effect on all microbial groups (p<.0001): meat from BEDM breed presented higher (p<.0001) populations of mesophiles, lactic acid bacteria, Pseudomonas spp. and psychrotrophic bacteria, since its higher ultimate pH (means: 5.77 for BEDM vs. 5.58 for CGB) accelerated spoilage rate (p<.0001). While water activity and protein content were not found to modulate microbial deterioration (p>0.05), the growth of spoilage bacteria was found to be exacerbated by higher moisture (p<.0001) and higher ashes content (p<0.001). By contrast, a higher fat content retarded (p<.0001) the growth of spoilage bacteria in VP lamb meat. In order to extend the shelf-life of Portuguese-origin lamb meat, animal handling must be enhanced to minimise pre-slaughter stress, and a carcass classification system should be adopted towards the selection of fatter animals and chilled carcasses of optimal ultimate pH.