The opening, announced by Network Rail today (7 February), will give FCC commuters using platforms 9, 10 and 11 to and from Peterborough, Cambridge, King's Lynn, Hertford, Welwyn Garden City and stations in between, direct access to the new two-storey concourse.
There will be an easier route in and out of the station and new faster access via an escalator to the Tube lines below.
The concourse will boast a new ticket office/travel centre, and shops and cafes over two levels, as well as a pub.
FCC Customer Service Director Keith Jipps said: "For years our customers using, as most do, platforms 9, 10 and 11 in King's Cross, have had to push their way along platform 8 to reach the main concourse.
"From Monday, 19 March, all this will change when the new modern concourse opens, making life much better for our customers.
"We strive to give good customer service. This new concourse will create a station impressive enough to rival St Pancras International next door."