Pompey chairman Milan Mandaric is understood to have set his sights on the Blades boss after Thursday's sacking of Alain Perrin, although the South Yorkshire side are likely to pull out all the stops in a bid to keep him at Bramall Lane.
United chairman Kevin McCabe has already expressed his dismay that Portsmouth have gone public about their bid to snare Warnock, saying: "What I've read is improper. It can cause unrest and I don't like it."
As well as Robson, Mandaric's wanted list is also understood to include such names as Roy Keane, David Pleat and Ian Dowie.