Council of Chalcedon, Canon XII

It has come to our knowledge that certain persons, contrary to the laws of the Church, having had recourse to secular powers, have by means of imperial rescripts divided one Province into two, so that there are consequently two metropolitans in one province; therefore the holy Synod has decreed that for the future no such thing shall be attempted by a bishop, since he who shall undertake it shall be degraded from his rank.  But the cities which have already been honoured by means of imperial letters with the name of metropolis, and the bishops in charge of them, shall take the bare title, all metropolitan rights being preserved to the true Metropolis.

Translation: Henry R. Percival, The Seven Ecumenical Councils, NPNF II.14 (1900), via CCEL.