Although Immediate Sales is a subsidiary of M&C Saatchi, Tory officials said it would not operate as a tool of Maurice Saatchi, the party's co-chairman.
One senior Tory said: "We will obviously draw on Maurice's expertise. But we want a relationship with an agency acting independently."
The Tories have signed up Media Planning Group to handle their media buying. But they have severed links with Vallance Carruthers Coleman Priest, which had been tried out by Will Harris, who quit as the party's director of marketing this summer.
Immediate Sales was involved in rebranding the Tories at their annual conference in Bournemouth this week, where it unveiled a new version of their "torch" logo and the slogan "Timetable for action", setting out what a Tory government would do. Some commentators mocked the logo, calling it "Stalinist". But Tory bosses said they were delighted with it.