Transport for London (TfL) has apologised after severe delays on the Northern line have entered a fifth day.

A signalling equipment failure at Stockwell station in south London has meant that since Sunday, service controllers have had to "manually talk trains through this part of the network".

The BBC understands that TfL has not yet identified the cause of the failure but letters have been sent to councils, businesses and other groups which TfL works with.

Nick Dent, TfL's director of customer operations, said: "Our engineers are working hard to identify and fix the fault, including deploying advanced test equipment, and we will do all we can to restore a good service as soon as possible."

He added: "We apologise to customers affected by delays on the Northern line since Sunday.

"We have identified a fault with the signalling equipment around the Stockwell area that means our service controllers need to manually talk trains through this part of the network.

"We are advising customers to allow extra time for their journeys, and to check before they travel using TfL journey planner or the TfL Go app."