The TCA (EU-UK Trade and Copperation Agreement) establishes a very comprehensive institutional framework with Partnership Council and diverse Committees having partly substantial decision-making powers for the development of the TCA. These considerable public functions prompt legitimacy concerns as to their democratic control, which this article explores in detail. It will be shown that the exercise of public powers by TCA treaty bodies meets with a sobering legal situation regarding democratic control mechanisms over treaty body decision-making at different levels. Thus, from a constitutional perspective, the legal and legitimate transfer of powers requires additional safeguards as to their democratic legitimacy. Solutions for better control of treaty body decisions by parliaments must be developed at several levels simultaneously.