The emergency services have been called after a passenger was taken ill on a train at Royston station.
As a result, the line from Royston to Cambridge is currently blocked.
Trains on the route from King's Cross may terminate early at Hitchin or Stevenage, or face alterations or delays to or from Cambridge.
Great Northern has said that trains can still run through Royston on a diversionary route, and will do so where possible.
Using these half lines will cause delays throughout the station, so passengers are advised to allow 10 extra minutes to complete their journey.
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