Severe delays on the Piccadilly line are limiting access to Heathrow this afternoon.
A signal failure on the Tube line is causing travellers major delays.
Transport for London (TfL)'s Piccadilly line tweeted there is no service between Northfields and all Heathrow terminals and severe delays between Acton Town and Northfields.
One traveller asked when the issue would be resolved via Twitter, to which the Piccadilly line said it couldn't predict.
Heathrow Connect services from Paddington will be accepting London Underground tickets while the disruption is ongoing, the line said.
TfL recently said there wasn't a single weekday in December when the Piccadilly and Central lines had the required amount of trains during peak evening times.
The Piccadilly line requires 79 trains during peak weekday times, yet for the majority of the month it ran with considerably fewer. Early in the month it was using few as 52 trains while it repaired trains damaged by high levels of leaves on the tracks.