In UK airspace:
4.4 Operation of Aircraft When ACAS II is Unserviceable
4.4.1 The current TCAS II Minimum Equipment List permits TCAS II equipped aircraft to operate for up to 10 days with the equipment out of service. This position will be kept under review.
4.4.2 Due to the safety benefits arising from TCAS operations and the collaborative way in which it arrives at collision avoidance solutions any aeroplane with an unserviceable transponder as well as an unserviceable TCAS will not be permitted in UK airspace for which mandatory carriage of a transponder is required.
(Source: UK AIP)