The identity of 51 isolates of Armillaria from 15 Quercus robur trees in poor health, and a single healthy tree, at nine sites in England, was determined using multilocus sequence analysis (MLSA) of three gene regions. Sequences of the ITS-1, IGS-1 and EF-1α gene regions were obtained by polymerase chain reaction (PCR) amplification and sequencing, and phylogenetic trees were generated based on maximum likelihood and Bayesian inference of phylogenies. Four Armillaria species were isolated: Armillaria gallica, A. mellea, A. ostoyae and A. tabescens. Armillaria gallica was most frequently isolated (40/51 isolates), but only from woodland trees. Armillaria mellea was isolated infrequently (3/51), from garden trees; A. tabescens was isolated infrequently (4/51), from trees either in a garden or a parkland location. Armillaria ostoyae (4/51 isolates) was co-isolated with A. gallica, raising interesting questions about the synecology of these species, suggesting that more thorough investigations are required to detect all species present on a single host. The distribution of these Armillaria species in Britain and historical information about them on oak are described. It is concluded that further studies are necessary to determine the role of Armillaria in oak declines; A. gallica should be a key focus, but investigations should include polymicrobial interactions with other microorganisms, including other Armillaria species.