Improvements include a new 5,000 sq metre concourse - over twice the size of the existing one with new shops and restaurants.
There will also be new transport links with international and domestic services from St Pancras, as well as London Underground.
The Grade 1 listed façade of the station will be restored by demolishing the 'temporary' current single-storey front to the station.
The train shed and platforms will be refurbished and the roof replaced. A new platform at the station will also increase capacity.
The total cost will be £400M which includes creation of a £6M major new public square, £175,000 for covered cycle parking spaces, to encourage cyclists to use the station, £750,000 for Camden Borough Council to implement further local improvements in the surrounding area, and a commitment that 10% of the station's energy needs will be met from renewable sources, reducing its carbon footprint by up to 20%.