In the interview, Sorrell said if the planned merger went ahead, it would leave TNS and the Germany-based GfK with surplus staff in France and Germany. He also stated that WPP would still be interested in an acquisition if TNS merged with GfK.
In response to Sorrell's comments, Wuebbenhorst wrote to GfK staff attacking Sorrell for using "morally irresponsible" tactics, and claiming that the merger would generate additional jobs.
According to the Sunday Telegraph's translation of Wuebbenhorst's memo, he said: "Our competitor WPP deliberately uses the tactic to create anxiousness, particularly against our colleagues, and this is morally extremely irresponsible.
"It is obvious that two companies of such strengths and expansion, like GfK and TNS will need, in future, more rather than less employees."
TNS and GfK believe they can find €102m (£80m) in cost savings, which is higher than original estimates by analysts.
Sorrell told the Sunday Telegraph, having read Wuebbenhorst's comments: "He seems to suggest they move to China or Latin America. Where will GfK-TNS find synergies of €102m by year three?"