Peter Thompson, founder and CEO of Roundtable, has also been appointed group chairman of CI Group.
David Watt, managing director of Corporate Innovations and CEO of CI Group, said that the two agencies coming together will allow the business to provide a "broader, full-service integrated marketing communication option for clients".
He said: "We’ve worked closely with Roundtable for a number of years, with Peter acting as a non-exec director for us and joining our leadership team earlier this year.
"Acquiring Roundtable accelerates our long-term expansion plans that broaden our client offer and appeal to a wider client base. Together, we now have all the skills necessary to create multi-disciplinary integrated campaigns."
Peter Thompson added: "The CI Group structure allows the two companies to continue to work independently and do what they do best, while providing the collaborative framework for them to co-exist and work together effectively and efficiently when the need arises. My role as Group chairman means I’ll be perfectly placed to identify and nurture joint business opportunities."
Corporate Innovations and Roundtable will continue to work from their respective offices in Oxfordshire and London's Soho. Peter Thompson will continue as CEO of Roundtable.