Smoke has been spotted in the tunnel between Tottenham Court Road and Goodge Street leading to a mass evacuation of the station.
Footage shared online showed passengers leaving Tottenham Court Road in droves after the evacuation order was given on Thursday morning.
Transport for London told The Mirror that smoke was detected between the two stations, and that fire crews were currently in the tunnels searching for the cause.
Just before 11.30am, TFL announced the station had been reopened after fire crews addressed some smoke on the tracks. According to reports, the Northern Line was shut between Leicester Square and Camden Town with minor delays at the rest of the stations because of the fire alert.
Last week, King's Cross Station had to be evacuated amid reports of passengers feeling "dizzy from fumes". Passengers were rushed out of the central London train station shortly before 4pm last Saturday as London Fire Brigade crews headed to the scene. As a loudspeaker message demanded that customers 'leave the station immediately' due to a 'reported emergency', one member of the public claimed that they felt 'extremely dizzy' having apparently inhaled unidentified fumes.
Mick Lynch warns rail staff could still be on strike this time next yearOne video taken from inside the Tube station underground showed worried looking members of the public covering their mouths and noses as they headed for the exit, a Tanoy announcement telling them to evacuate as they did.
The fire brigade initially revealed they'd responded to an activated fire alarm and it was not yet clear if the emergency was a false alarm. Transport for London was unable to offer any more information.
The closure of the station caused serious transport disruption, with a number of trains due to depart and arrive at King's Cross significantly delayed. One would-be-passenger at the station described a "scramble for buses" as evacuation took place as "emotions ran high".
Unconfirmed reports suggest that a flare was set off in the Underground section of the station. A police officer at the scene told The Sun: “Someone has set off a flare in the Underground and we’ve had to shut down the area. Firefighters are down there now. We don’t know when we’ll open the Underground again."