A prototype silicon-tungsten electromagnetic calorimeter (ECAL) for an international linear collider (ILC) detector was installed and tested during summer and autumn 2006 at CERN. The detector had 6480 silicon pads of dimension 1 x 1 cm(2). Data were collected with electron beams in the energy range 6-45 GeV. The analysis described in this paper focuses on electromagnetic shower reconstruction and characterises the ECAL response to electrons in terms of energy resolution and linearity. The detector is linear to within approximately the 1\% level and has a relative energy resolution of (16.5 +/- 0.14(stat) +/- 0.4(syst))/root E(GeV) circle plus (1.07 +/- 0.07(stat) +/- 0.1 (syst)) (\%). The spatial uniformity and the time stability of the ECAL are also addressed. (C) 2009 Elsevier B.V. All rights reserved.