An underground ticket hall which will serve passengers using the new Eurostar terminal and those heading to the 2012 Olympic Games has been completed.
The ticket hall will serve people travelling to the Olympics
The Western Ticket Hall is the latest phase in a development to transform King's Cross into a transport hub.
Stair-free access, extra routes to street level and bigger subways will allow thousands more people to pass through the north London Tube station.
A second ticket hall is due to be completed by 2010.
Passenger trains on the Channel Tunnel Rail Link are expected to start running from the new terminal at St Pancras next year.
Services between St Pancras and Stratford International station, serving the Olympic Games site in east London, are also due to begin in 2007.
The work at King's Cross is part of Transport for London's £10bn investment programme and was funded by the Department for Transport.
Mayor of London Ken Livingstone and the Secretary of State for Transport Douglas Alexander toured the ticket hall on Thursday ahead of its opening on Sunday.
Mr Alexander said it will be a fitting gateway to London for passengers from the UK and abroad and will help to create a fully integrated transport hub in the heart of the capital.
Mr Livingstone added: "The station is state-of-the-art, providing a blueprint for the future and a building fit for a world-class city hosting the Olympic Games."