The takeover sees CTI become the European arm of sister CSS agency GEM, which the network bought last year, and means the end of the CTI brand.
CTI - which has worked for Vodafone on its sponsorship of English cricket and four-year deal with Manchester United - was founded in 1981.
Alongside CTI's sports and sponsorship clients, the agency's consumer arm looks after brands including The Times and UGC Cinemas.
Consumer marketing agency GEM handles clients including Coca-Cola.
GEM Europe will be headquartered in CTI's Guildford offices and led by former CTI managing director Kate Waterfall, as chief operating officer, and CTI founder Marcus Robertson, who becomes non-executive chairman.
Robertson said they plan to expand staff numbers by more than 50 per cent to around 75 or 80 people by the end of next year. There are also plans to open six offices on the continent, largely by acquisition, to cover markets including Benelux, Iberia, Germany and Scandinavia.
Roberston, who worked for CSS as an account executive before launching CTI, said: 'CSS doesn't have a mainstream consultancy division, it's mainly a sales operation.
The deal will also see CTI deputy MD Giles Morgan become client services director for GEM Europe. GEM vice-president North America Chris Carroll will join the European arm on a two-year secondment.