This is from Doc 4444, 15th edition (there have been a few revisions since this was published in 2007 but I don't think they've affected the following):
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8.6.5.2 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 the point where the pilot will resume own navigation. When necessary, the relevant minimum vectoring altitude shall include a correction for low temperature effect.
Note 1.— When an IFR flight is being vectored, the pilot may be unable to determine the aircraft’s exact position in respect to obstacles in this area and consequently the altitude which provides the required obstacle clearance. Detailed obstacle clearance criteria are contained in PANS-OPS (Doc 8168), Volumes I and II. See also 8.6.8.2.
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Cheers
Quintilian