Train services to and from St Pancras Station face long delays today after an "incident" at the station.
Photos showed scores of commuters stuck at the central London hub as officials investigated the disturbance.
Twitter users report the whole station has been evacuated after it emerged the fire alarm had gone off.
Services are still likely to affected until 3pm.
Southeastern Rail said on Twitter: "We've been advised that this is due to emergency services dealing with an incident at #StPancras meaning that presently no trains can arrive or depart from the station.
"London Underground are accepting tickets between our London Terminals to help complete your journey."
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A spokeswoman from the British Transport Police confirmed the incident was related to a fire alarm.
The station serves areas in the east of England, including Cambridgeshire, Northamptonshire, Bedfordshire and Hertfordshire.
It also serves areas in southeastern England.
