Rail passengers are being warned of “major disruption” after overhead electric wires were damaged in north-west London.
Trains are currently unable to run between Luton and London St Pancras and disruption is expected until 12pm, National Rail said.
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.