Vote No is a non-party campaign formed by some of Britain's most successful entrepreneurs to garner support for a "no" vote in the forthcoming referendum on the European Constitution.
Its founders include Next's chief executive, Simon Wolfson, the P&O Group's chief executive, Robert Woods, and Carphone Warehouse's co-founder, David Ross.
Vote No awarded WCRS the business after a series of meetings with agencies.
It will work with its sister agencies Dave, Meme and Personal to create a multimedia campaign designed to convince key voter groups of the dangers of the European Union constitution.
Stephen Woodford, a founding partner of WCRS, said: "Being against the draft constitution does not mean being anti-European. We will be presenting a 'no' vote as a positive thing - a vote for keeping control and an opportunity for the kind of reform Europe urgently needs."
Vote No's chief executive, Alex Hickman, added: "We have set ourselves a multimillion-pound fundraising target and will spend what it takes to counter the Government-funded 'yes' campaign."