The CALICE collaboration is preparing large scale prototypes for highly granular calorimeters for detectors to be operated at a future lepton collider. During the years 2011-2012 a prototype of a silicon tungsten electromagnetic calorimeter will be assembled which in terms of dimensions and layout meets already most of the requirements given by the lepton collider physics program and hence the detector design. In particular the front end electronics will be embedded into the layer structure of the calorimeter and have to fit within alveolar layers with less than 1 cm in height. In this contribution the design of the prototype is presented and the steps towards the realisation will be outlined. At the conference results of beam test campaigns with first layers of the prototype will be presented.