Police have been seen entering the station, with seven platforms blocked off.
Severe delays to rail services have been reported.
One Twitter user reported seeing police dogs enter the station.
Great Northern Rail tweeted to travellers: “A problem currently under investigation is disrupting services to/from Kings Cross.”
They added: “Police are dealing with an incident at Kings Cross.”
“The station is shut and we can’t run to/from Kings Cross until further notice.”
They then quickly posted: “Kings Cross station is open and we can to/from the station as normal again.
“Some residual delays are possible, so please check before you travel.”
More to follow…