Commuters using the Piccadilly line are facing the third day of disruption this week.
There are severe delays on the line between Acton Town and Cockfosters this morning, due to a signal failure at Ravenscourt Park.
Transport for London (TfL) said customers may experience disruption on the rest of the line.
Piccadilly line: Severe delays are occurring between Acton Town and Cockfosters only whilst we repair a signal failure at Ravenscourt Park. Customers may also experience slight disruption on the rest of the line due to this. https://t.co/AWP1jcM1cE— Piccadilly line (@piccadillyline) November 22, 2017
"We apologise to our customers and are working to correct the problem," a TfL spokesperson said.