Objectives Multiresistant Escherichia coli with CTX-M-15 extended-spectrum ß-lactamase (ESBL) are widespread in the UK. We examined their phylogenetic groups and virulence factors. Methods Clinical E. coli isolates (n = 114), collected between 2003 and 2006, were phylogenetically grouped and screened by PCR for 33 virulence factor genes. They included representatives of the five major UK epidemic E. coli strains with CTX-M-15 enzyme, as well as non-clonal isolates with CTX-M or other types of ESBLs. Results All representatives of the epidemic E. coli strains belonged to the virulent extra-intestinal phylogenetic group B2, as did 60% (34/56) of the non-clonal isolates with CTX-M-15-like enzymes and 75% (15/20) of those with non-CTX-M ESBLs. Half of those with CTX-M-9-like enzymes belonged to virulence group D. Within phylogenetic group B2, the prevalence of most virulence factors was comparable among clonal and non-clonal isolates with CTX-M enzymes, and among those with non-CTX-M ESBLs. The most frequent virulence genes were PAI, fimH, fyuA, iutA, kpsMTII, K5, traT, uidA and usp. Among the five epidemic clones, afa/draBC was specific to strain A, whereas P fimbriae were only detected in strain D, and only representatives of the B–C–E group specifically harboured sfaS, kpsMTII and K5. However, afa/draBC was also found in 30% of non-clonal isolates with CTX-M ESBLs, and no virulence gene was unique to the epidemic strains. Conclusions Most E. coli with CTX-M ESBLs belonged to virulent phylogenetic groups, mainly B2. The successful epidemic strains did not appear more virulent, but iutA and fyuA were significantly more prevalent among these than in non-clonal isolates also belonging to phylogenetic group B2. The most successful clone with CTX-M-15 enzyme (A) differed from other epidemic clones in harbouring afa/draBC, but this was also found in non-clonal isolates with CTX-M-15 enzyme.