Abstract Background Chestnut blight, caused by Cryphonectria parasitica , is controlled in many European countries by the naturally occurring mycovirus Cryphonectria hypovirus 1 (CHV‐1). During surveys of recently identified chestnut blight outbreak in England, CHV‐1 was detected in several individuals of the pathogen isolated from affected trees. We investigated two of these CHV‐1‐infected isolates (L‐6 and Db‐1) as potential biocontrol agents for deployment in the UK comparing their virulence against virus‐free (M1275) and hypovirulent (M784) European isolates by inoculating sweet chestnut seedlings. Results Both the European CHV‐1 M784 hypovirulent isolate and UK L‐6 isolate formed significantly smaller lesions in sweet chestnut seedling bark than the other three isolates (Db‐1, and virulent isolates FTC121 and M1275). The highest virus concentration was detected in isolate M784, followed by L‐6, with the lowest concentration in isolate Db‐1. White colony colouration indicative of hypovirulence was common in colonies re‐isolated from smaller lesions, and the same isolates also tended to be slower growing in culture, have a higher virus concentration, and caused less epicormic growth and fewer stromata to be present in plants. L‐6 and Db‐1 virus sequences, respectively, matched the virus haplotype E‐5 detected previously in Switzerland and a mutation of the same subtype I haplotype. Conclusion Isolate L‐6 could potentially act as biocontrol for chestnut blight outbreaks in the UK but further laboratory and field experiments are needed. © 2019 Crown copyright. Pest Management Science © 2019 Society of Chemical Industry