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High speed trains through Kent were running slow after a trespasser was spotted on the track between Stratford and St Pancras.
The person was spotted by a driver at about 3pm, entering the London Tunnel at the St Pancras end.
Three British Transport Police officers got on the following service and stopped the train at the tunnel to search for the trespasser.
The person was found and taken to St Pancras by officers.
All trains using the line have been told to reduce speed, and Southeastern is reporting delays of up to 45 minutes.