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 17:00 BST, just before the peak commute.
A speed restriction between St Albans and West Hampstead Thameslink is adding to the disruption.
Thameslink said services through London St Pancras International, Farringdon, City Thameslink and London Blackfriars were severely affected.
“Train services running between these stations may still be cancelled, delayed or revised,” it added.
“We advise you to avoid travelling on Thameslink routes wherever possible.”
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.”