Damaged electrical wires are causing major disruption on rail services between London St Pancras and Luton Airport Parkway.
ThamesLink said the damage to overhead wires at Elstree and Borehamwood in Hertfordshire meant some services would be cancelled and others delayed up to 30 minutes.
The issue is disrupting trains on the London St Pancras International, Nottingham, Sheffield and London St Pancras International, Luton Airport Parkway and Corby route.
Disruption was expected until the end of the day, the rail operator said.
