Damage to the overhead electric wires between Harpenden and Bedford are causing disruption to journeys between London St Pancras International and Bedford.
Thameslink has said because of the damage trains may be cancelled or delayed for the rest of the day.
During the morning peak there will be significant reductions to services.
Services that run to or from Sutton, Brighton, Peterborough, Horsham and Cambridge will be able to run, however some cancellations on these routes are likely.
There will be a reduced service running on the available lines for journeys from Bedford to London.
Ticket acceptance will take place on Thameslink and Great Northern services running from Peterborough towards London.
Plus London Northwestern Railway services between Bedford and Bletchley and St Albans Abbey to Watford Junction.
There are also a number of bus services on which tickets will be accepted.
For more information visit Thameslink's website.
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