Transport for London said the disruption was a result of a "temporary absence of train operators".
Update: Minor delays due to a temporary absence of train operators.— Circle line (@circleline) December 7, 2016
Severe delays on the Piccadilly Line are still ongoing, with a large portion of its fleet of trains out for repairs.
Severe delays while we carry out repairs to our trains, therefore, a reduced service is operating on the line.— Piccadilly line (@piccadillyline) December 7, 2016
Disruption on the line is now expected to continue until after Christmas, as Tube bosses struggle to get repaired trains back into operation quickly enough.