Almost 50 UN human rights experts have condemned Israel's plan to annex parts of the occupied West Bank, calling it a "vision of a 21st Century apartheid".
Such a move would violate international law and leave what would amount to "a Palestinian Bantustan", they warned.
Prime Minister Benjamin Netanyahu says in July he may start the process of applying Israeli sovereignty to Jewish settlements and the Jordan Valley.
Such a move was effectively green-lighted by Donald Trump's peace plan.
Mr Trump's Vision for Peace, released in January, also envisages a Palestinian state in the remaining 70% of the West Bank, all of Gaza, and with its capital on the fringes of East Jerusalem.
The Palestinians - who claim all of the West Bank, Gaza and East Jerusalem - have dismissed the plan as biased towards Israel and a denial of their rights.
Israel has occupied the West Bank and East Jerusalem since the 1967 Middle East war. It pulled its troops and settlers out of Gaza in 2005, but the UN says its occupation there has not ended.
More than 600,000 Jews live in about 140 settlements in the West Bank and East Jerusalem. Most of the international community considers the settlements illegal under international law, though Israel and the Trump administration dispute this.
What do the UN experts say?
"The annexation of occupied territory is a serious violation of the Charter of the United Nations and the Geneva Conventions, and contrary to the fundamental rule affirmed many times by the United Nations Security Council and General Assembly that the acquisition of territory by war or force is inadmissible," said the 47 experts, who are appointed by the UN Human Rights Council.
They added that the Israeli occupation was already a "source of profound human rights violations against the Palestinian people", and that they "would only intensify after annexation".
"Israel has recently promised that it will maintain permanent security control between the Mediterranean and the Jordan River. Thus, the morning after annexation would be the crystallisation of an already unjust reality: two peoples living in the same space, ruled by the same state, but with profoundly unequal rights. This is a vision of a 21st Century apartheid."
The experts noted that Israel had annexed occupied land in East Jerusalem in 1980 and the Syrian Golan Heights in 1981, and that on both occasions the UN Security Council had condemned the actions but taken "no meaningful countermeasures". "This time must be different," they added.
There was no immediate comment from the Israeli government.
However, an unnamed Israeli diplomatic source told the Times of Israel that the statement "does nothing to help finding a solution to the conflict or to create a constructive dialogue between the parties". The source also said that some of the experts "have nothing to do with the region or the rights the statement deals with".
The secretary general of the Palestine Liberation Organisation, Saeb Erekat, welcomed the experts' statement as "a reminder for the international community of its responsibilities, of the gravity of the situation and of the urgency to implement accountability measures to end the illegal colonial-settlement enterprise".
What is the Israeli government's plan?
Prime Minister Netanyahu says he wants to "apply Israeli sovereignty" to the parts of the West Bank which contain the Jewish settlements, as well as most of a swathe of land along the West Bank's boundary with Jordan, known as the Jordan Valley.
Palestinians in the Jordan Valley will be exempt from annexation, and reports say the same exclusion will to apply to Palestinians in other annexed parts of the West Bank.
The areas earmarked for annexation (the precise contours of which are being mapped by Israel and the US) may comprise about 30% of the West Bank, according to reports.
Mr Netanyahu might initially act to annex just the settlements, which could amount to only 3%, and the remaining 27% later on once the boundaries are agreed with Washington.
Why does it intend to act now?
Israel claims historical and religious rights to the West Bank as the ancestral land of the Jewish people. It also says its presence there - especially in the Jordan Valley - is strategically vital for its self-defence.
It says settlements are not an obstacle to peace and that they would remain part of Israel under any peace deal with the Palestinians, whether they are annexed now or not.
Mr Netanyahu has long championed the settlements and through annexation wants to remove any doubt as to their fate, something which strongly appeals to his political base.
Until recently, Mr Netanyahu would have faced solid opposition among the international community to such a move. However, President Trump's plan for peace between Israel and the Palestinians allows for Israel to "incorporate" all the settlements - something no previous US administration had countenanced.
It is possible that Mr Netanyahu wants to get it done before the US elections in November in case Mr Trump's rival Joe Biden - who opposes annexation - is elected and reverses US policy.