Extended-spectrum beta-lactamase (ESBL)- and plasmid-mediated cephalosporinase (AmpC)-producing Enterobacterales (ESBL/AmpC-E) are an increasing healthcare problem in both human and veterinary medicine. The aim of this study was to investigate the possible sharing of ESBL/AmpC-E strains between healthy companion animals and humans of the same household in Portugal (PT) and the United Kingdom (UK). In a prospective longitudinal study, between 2018 and 2020, faecal samples were collected from healthy dogs (n=90), cats (n=20) and their cohabiting humans (n=119) belonging to 41 PT and 44 UK households. Samples were screened for the presence of ESBL/AmpC-E and carbapenemase-producing bacteria. Clonal relatedness between animal and human strains was established by using REP-PCR fingerprinting method, followed by whole-genome sequencing (WGS) of selected strains. ESBL/AmpC-E strains were detected in companion animals (PT=12.7%, n=8/63; UK=8.5%, n=4/47) and humans (PT=20.7%, n=12/58; UK=6.6%, n=4/61) in at least one timepoint. REP-PCR identified paired multidrug-resistant ESBL/AmpC-producing Escherichia coli strains from companion animals and owners in two Portuguese households (4.8%) and one UK household (2.3%). WGS analysis of nine E. coli strains from these three households confirmed that interhost sharing occurred only between the two animal-human pairs from Portugal. Three shared strains were identified: one CTX-M-15-producing E. coli strain in a cat-human pair (O15-H33-ST93) and two CTX-M-15- and CTX-M-55/CMY-2-producing E. coli strains, in a dog-human pair (O8:H9-ST410 and O11:H25-ST457, respectively) at different timepoints. These E. coli clonal lineages are human pandemic, highlighting the role of companion animals living in close contact with humans in the dissemination and persistence of antimicrobial resistance in the household environment.