The disruption is due to a power supply failure that has caused all Thameslink services on that part of the line to be cancelled or revised.
Thameslink trains between Rainham or Gillingham in Kent and St Albans City or Luton are now terminating early at Dartford and will not travel further towards central London.
Stations between Dartford and London Bridge are not being served by Thameslink today, including Slade Green, Abbey Wood, Plumstead, Woolwich Arsenal, Charlton, Westcombe Park, Maze Hill, Greenwich, and Deptford.
National Rail said: “Engineering work taking place between Dartford and London Bridge requires the railway power on this section to be switched off.
“Due to a fault with the electricity substation in the South Bermondsey area, which provides electricity to the railway, this line can’t be isolated, this means that the power switch off will affect all Thameslink routes through the area.”
Passengers heading to or from the affected stations are being advised to use alternative services.
Tickets can be used at no extra charge on London Buses on routes between Dartford and London Bridge and Southeastern services on any reasonable route between Rainham or Gillingham and London Victoria or London Bridge.
Passengers may also use the London Underground where necessary, and the Elizabeth line between Farringdon and Abbey Wood.
Journeys are expected to take up to 30 minutes longer, and travellers may need to change trains more than usual.
Commuters are urged to check journey planners and station departure boards before setting off, and to allow extra time to reach their destinations.
Live updates and travel advice are available on the Thameslink app and the National Rail Enquiries real-time Journey Planner.
Compensation may be available for passengers delayed by the disruption.
Travellers should keep hold of their tickets and make a note of their journey details in order to support any claim.
The disruption is expected to continue until at least the end of today, Saturday, May 2.
