The steel and glass dome, which weighs 1,700 tonnes, was designed by architect John McAslan to complement the rest of the Grade I listed station.
Commuters will be able to walk through the new wing from next Monday.
Mayor of London Boris Johnson and transport secretary Justine Greening officially opened the station yesterday alongside Network Rail chief David Higgins.
The new western concourse, at a cost of £550m, is part of Network Rail’s £2.5bn, 10-year upgrade of the King’s Cross St Pancras and surrounding area, which runs until 2013. More than 45m people travel through King’s Cross station a year.