The £550 million scheme is described by Network Rail as being "the biggest transformation in the 160 year history of the Grade-I listed station ... [marking] the completion of yet another key transport upgrade for the Olympic and Paralympic Games".
The opening takes total investment in transport infrastructure in the King's Cross area to £2.5 billion in the last ten years.
The new concourse will open to the public on Monday and includes a new glass roof structure which creates a space for passengers that is three times the current concourse. It has improved links to both the London underground and St Pancras station and better passenger facilities.
The project has been described by Network Rail as a "catalyst for economic growth" with high profile businesses expected to move into the new area and further phases of development to the station expected to be completed in Autumn 2013.
The project is the result of a partnership between Network Rail, train operators, the local council, architects, suppliers and English Heritage.
