The tree is on the line between Hurst Green and East Grinstead and is blocking all lines.
This means that Lingfield, Dormans and East Grinstead currently have no trains departing at all.
Disruption is expected until at least 7pm.
A spokesman for Govia Thameslink Railway, the operator in charge of Southern services said: “You will need to use another route for your journey, or travel later today, once the line reopens. Please leave at least one hour of extra time to get to your destination.”
Ticket acceptance is in place on the following:
- Metrobus services on any reasonable route, including services between East Grinstead and Three Bridges, for alternative train connections to/from London, and on the limited bus services via Lingfield and Dormans
- Southdown PSV buses between Oxted and Redhill, and between Redhill and Gatwick Airport / Crawley for Metrobus connections
- Alternative Southern and Thameslink services via Three Bridges