The agency picked up the account from Ogilvy & Mather without a pitch.
Its appointment follows GSK's decision to consolidate the brand's advertising into one network. Havas' Arnold handles the account in the US.
Last year, the brand spent £3.5 million on advertising in the UK, according to Nielsen Media Research.
Mark Dickinson, the GSK category director for smoking control, said the decision was "no reflection on the work O&M produced", and added that the company was looking for better consistency across its brand advertising.
WCRS is a wholly owned Havas subsidiary, although it is in the process of orchestrating a management buyout, the details of which have yet to be revealed. Nevertheless, Havas is expected to retain a significant minority stake hold.
Stephen Woodford, the chief executive of WCRS, said: "Arnold has done a great job with these brands in the US and we will be working closely with them to develop work in the UK and Europe.
"This is a really exciting win for us as we are passionate about anti-smoking as an agency."
WCRS does not have a European network, but will coordinate work with Arnold network agencies in other markets including devarrieuxvillaret in France and Ata De Martini in Italy.
Woodford said the agency would be working on the brief over the next year, with the first activity expected to roll out from early next year in France, Italy and the UK.
Ogilvy & Mather is working out its notice on the account.