Godfrey said no shareholders had been informed of the deal, which will create a new holding firm, MDA Consulting, to take over from MDA Group. His letter asked: "Why have shareholders not been brought into the debate to explore ways in which the company might be saved?"
MDA claimed it had handed over the letter to administrative receiver Robson Rhodes.
In a statement, Johnston defended his decision, taken last month. He said: "The solution reached ensured that the current clients of MDA continue to receive an ongoing and excellent service, while securing the jobs of our 160 staff."