Train lines are currently blocked between Stevenage and Peterborough and Hitchin and Cambridge, after damage to overhead electrical wires was discovered earlier this afternoon.
Customers are advised not to use this route, as disruption is expected until the end of the day.
Thameslink Railway has said the damage is “substantial” and “will not be fixed today”.
It also said: “We are fairly certain that services between Hitchin and Peterborough will not resume tonight, with work planned to take place overnight.
“It is also very likely that early morning services will also be affected.”
Journey times may be extended by up to two hours, and passengers are advised to seek alternative routes.
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