On 18 December 2020, a Bombardier Challenger descending on an ILS approach into Sion in visual meteorological conditions (VMC) within the Sion terminal control area (TMA) was advised of unknown VFR traffic on a potentially conflicting track and working a different frequency. Minimum separation was 100 feet vertically and just over 1 nm laterally before the Challenger response to a TCAS resolution advisory (RA) increased vertical separation despite the other traffic also initially attempting visual separation by climbing. The Class ‘D’ airspace Sion TMA was inactive and therefore downgraded to Class ’E’. The investigation concluded that procedures which prevented activation of the TMA at short notice were outdated.