Klebsiella pneumoniae (K. pneumoniae) cultures from a hospital-wide outbreak in the UK, which lasted for over 12 months, were sequenced. We sought to sequence and genetically characterise the outbreak strain. Antibiotic Susceptibility Testing (AST) was performed on 65 K. pneumoniae isolates saved from the outbreak. All isolates were sequenced using the Oxford Nanopore Technologies (ONT) MinION flowcell: 10 isolates, including the isolate with the earliest collection date in 2017, were additionally sequenced on the NovaSeq 6000 platform to build high-accuracy nanopore-illumina assemblies. Among the sequenced strains, 60 were typed as ST628. 96.6% (n = 58/60) ST628 strains harboured a large ~247-kb FIB(K) plasmid carrying up to 11 antimicrobial resistance genes, including the extended-spectrum beta-lactamase (ESBL) gene, blaCTX-M-15. Clonality between the outbreak isolates was confirmed using single nucleotide polymorphism (SNP) typing. The outbreak strains were phylogenetically related to clinical ST628 strains identified in 2012, 6 years prior to the outbreak. A rare ESBL K. pneumoniae K2 ST628 strain harbouring a multi-drug resistant (MDR) plasmid encoding the ESBL gene blaCTX-M-15 was detected across multiple independent wards during the protracted nosocomial outbreak. Surveillance of this strain is recommended to prevent future nosocomial outbreaks.