“Major disruption” is expected at one of London’s busiest train stations and could last until at least midday, National Rail said.
Rail passengers have been told to check their journeys as trains are currently unable to run between Luton and London St Pancras.
The disruption, expected to last until 12pm, has been caused after overhead electric wires were damaged in north west London.
Thameslink also advised customers to "delay travel" until 2pm.
East Midlands Trains, whose London terminus is St Pancras, has strongly advised customers not to travel to or from the capital on Sunday.
Passengers from Sheffield and Nottingham travelling to London are being advised to change at Doncaster and Grantham respectively for East Coast Main Line services, while passengers at Leicester are urged to travel to Nuneaton for a service to Euston.
Replacement buses from Luton to Edgware London Underground station are being arranged for East Midlands Trains customers, National Rail said.
There is a replacement bus service with "extremely limited capacity" covering Thameslink services, with passengers asked to delay their journey if possible.
