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.
Last edited by bookworm; 18th May 2011 at 10:29.
Reason: Edited to correct ref and add quote.