Services between Haywards Heath and Gatwick Airport have been disrupted due to an incident in the Balcombe area.
Major disruption is expected with services cancelled, delayed or revised.
Thameslink, Southern and Gatwick Express services will be affected by the disruption until around 13:00.
Passengers travelling on routes in and around London, and towards Brighton and Gatwick Airport, are advised to delay travel where possible to avoid most of the disruption.
Services have already been altered, and ticket acceptance will be in place.
Full details will be available online.
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