London's fleet of Routemaster buses are suffering from major problems with its Valence lithium-ion battery packs.
Irish bus manufacture Wrightbus, who supplied the buses, are refusing to reveal who manufactured the battery packs and the why they failed.
Transport for London (TfL), which runs the service, said the performance of the batteries on the earlier vehicles had caused concerns.
An improved battery design is being used on new buses and replacements.
Out of the 500 new Routemasters currently on the roads, 80 of them are running in diesel-only mode because of failed batteries, and 200 of the buses are due to have their batteries replaced, TfL told the BBC.
The first new Routemasters were introduced in February 2012, with problems with the batteries being flagged up as early as March 2015.