Here is a list of some songs that give me similar feelings that Portishead does. They are not exactly like Portishead, but IMHO definitely worth a try.
Massive Attack - Unfinished Sympathy, Massive Attack - Teardrop, Röyksopp & Alison Goldfrapp - Impossible, Röyksopp - Vision One, Saint Saviour - This Ain't No Hymn
Massive Attack is closer to Portishead, but they don't have female vocals on all of their songs. Röyksopp goes to a bit different direction but they have many songs with different female vocalists that might be something you like.