A European Commissioner cannot be removed from office by the member-state that nominated him or her.
A commissioner can be dismissed only by the European Court of Justice acting on a request from either the member-states as a whole, acting through the Council of Ministers, or the full Commission. The Treaty establishing the European Community provides in Article 160 that: "If any member of the Commission no longer fulfils the conditions required for the performance of his duties, or if he has been guilty of serious misconduct, the Court of Justice may, on application by the Council or Commission, compulsorily retire him."
Legal sources point out that the process is not automatic - the court has discretion - and that misconduct need not necessarily relate to the "performance of his duties".
The Treaty contains this provision as a guarantee of the collegiality of the Commission and to protect a commissioner from improper pressures from his member-state. However, it is difficult to conceive of a commissioner who would not agree to resign when asked to do so by his government, if the circumstances are unrelated to his work as a commissioner.