Carr writes that in 1967 Israel's then prime minister, Levi Eshkol, sought advice from Israel's leading authority in international law, Theodor Meron, as to whether Israel could settle civilians in the territories that the country found itself occupying as a result of the Six Day War. Meron's advice, according to Carr,
'was unequivocal, and today he sticks to it. He said: “Civilian settlement in the administered territories contravenes explicit provisions of the Fourth Geneva Convention.”...'Carr continues, inter alia:
'Indeed, the Fourth Geneva Convention would appear to leave no room for argument. It states: “The occupying power shall not deport or transfer parts of its own civilian population into the territory it occupies.”
Apologists for settlements try to argue that Article 49 bars the occupier only from “forced transfer” (my emphasis) of its civilians. This is not the interpretation accepted by the International Court of Justice or anyone else. The adjective “forced” does not appear in the convention.
I think I recognise a killer argument when I see one. The killer argument here is that Israel’s own legal authority, at the very start, told its government that settlements were illegal under international law.'Citing the statement of frum Israeli settler leader Daniella Weiss in this week's "Four Corners" hatchet job on Israel that religious settlements are intended to thwart the creation of a Palestinian state since “this land was promised to the Jewish nation by God” Carr also cites a BBC documentary by Louis Theroux called "The Ultra Zionists":
'religious settlers declared Palestinians an inferior race. “This is the Jewish homeland and there’s never been a Palestinian people,” declared one, standing on a property formerly occupied by Palestinians. In one blast they defied centuries of priceless Jewish liberal and humanitarian instinct.'And he warns:
"No one from the centre-Left of European politics is going to do anything other than repudiate this ultra-religious vision...."
The entire article, "West Bank settlements always illegal" (try googling that ;) ), is behind a paywall here
For arguments that settlements are in fact legal see here