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Welcome, in effect, to what will be Tate Modern 2, a brand-new Β£215m extension that will change the South Bank and the capital's skyline for good.

The 11-floor building, to be built on the south side of the London gallery, has been boosted by the announcement of Β£50m from the Department of Culture, Media and Sport.

Below Tate Modern

The new structure, designed by Swiss architects Herzog and de Meuron, will provide more space for the Tate's collection and help reduce overcrowding at what is already the world's most popular modern art museum.

You can watch as our cameras go deep below Tate Modern to see where the new development will begin, and to hear from Culture Secretary James Purnell and Tate director Nicholas Serota.

Fundraising continues now to raise the rest of the money, following Β£7m which has been donated already from the London Development Authority and a further Β£5m from banker John Studzinski.

It is hoped the extension will open in time for the 2012 Olympics in London.