The agency has also promoted Charles Billot to commercial director, with responsibility for a newly launched mobile marketing division and for developing its interactive TV offering.
Billot, who was previously the agency's head of new-business development, will run the London office and take responsibility for Soup Mobile, which was set up to offer services such as mobile blogs, gaming, mobile video and application development.
Soup already has experience in mobile marketing. In one recent project, the agency launched www.trainblog.co.uk, a site to provide travellers with a place to upload com-ments on their journey via their mobile phones.
Welsh will head a five-strong strategy team with a focus on building Soup's planning offering.