Severe delays are affecting the Metropolitan Line after a signal failure, spelling commuter chaos for those heading to work from west London.
The failure at Harrow-on-the-Hill means local buses are accepting tickets, said Transport for London (TfL).
The delays come just a day ahead of strikes expected to wreak havoc on Tube lines.
The Central Line and Piccadilly Line are both set to be out of action tomorrow amid Aslef and RMT union strikes.
Aslef said it is fighting for “fair treatment” at work, accusing TfL of failing to make any new proposals since a similar strike a month ago.
Nigel Holness, director of network operations for London Underground, called the strike action “totally unjustifiable”, saying Aslef wants a Central Line driver reinstated after being sacked for opening train doors in a tunnel.
“We always strive to run a reliable service for our customers but our commitment to the safety of both customers and staff is absolute and something we will never compromise on. I urge Aslef to call off this entirely unnecessary strike action which will only cause disruption to our customers.”