Rail commuters are facing major delays in London due to “multiple incidents”.
Damage to overhead wires between London St Pancras and Blackfriars blocked all Thameslink services during rush hour.
Services have now resumed but the operator – which runs trains across the South East – warned of “major disruption until the end of the day”.
A speed restriction in Borehamwood is adding to the disruption, affecting mainly south-bound trains from St Albans to St Pancras.
Thameslink said there was a “significantly reduced service” through central London stations, London Bridge, Gatwick Airport, East Croydon, Sevenoaks, Orpington, Rainham, St Albans, Wimbledon and Sutton.
“Please check customer information screens and journey planners prior to travelling, and listen to station and train announcements for up to date service information,” it said.
Power to railway lines between London St Pancras and Blackfriars had been switched off following “multiple incidents”, Network Rail said.
“Network Rail staff have been working overnight and this morning to repair the issue,” the company said.
“Services have now resumed but passengers should check their journeys before travelling as residual delays are expected.”
Another train operator, Great Northern, has tweeted in reply to a customer: “We are experiencing severe congestion on the line due to damage to the overhead wires in London Blackfriars which results in late notice cancellation of services.”