I replied on the ATC forum, but in haste gave the wrong document as a reference, sorry:
PANS ATM (Doc 4444) 8.6.5.2 (also reflected in 4.10.3 Note 3).
When vectoring an IFR flight and when giving an IFR flight a direct routing which takes the aircraft off an ATS route, the controller shall issue clearances such that the prescribed obstacle clearance will exist at all times until the aircraft reaches a point where the pilot will resume own navigation.