Secretary-general says such a move will be 'devastating' for hopes of fresh talks and an eventual two-state solution.
United Nations chief Antonio Guterres has called on Israel to abandon plans to annex parts of the occupied West Bank, saying such a move would be a "most serious violation of international law".
The UN secretary-general made the comments in a report to the Security Council on Tuesday, a day before the 15-member body holds its twice-yearly meeting on the Israeli-Palestinian conflict.
The government of Israeli Prime Minister Benjamin Netanyahu has said it could begin the annexation process from July 1.
In the document, Guterres said an Israeli annexation would be "devastating" for hopes of fresh negotiations and an eventual two-state solution.
"This would be calamitous for Palestinians, Israelis and the region," he said, adding that the plan threatened "efforts to advance regional peace".
Guterres's comments came a day after thousands of Palestinians protested in Jericho against the Israeli plans, in a rally also attended by dozens of foreign diplomats.
The Palestinian leadership proposed last week a plan that seeks to create a "sovereign Palestinian state, independent and demilitarised", with East Jerusalem as its capital. It also leaves the door open to border modifications between the proposed state and Israel, as well as exchanges of land equal "in size and volume and in value - one to one".
The Palestinian proposal came as a response to US President Donald Trump's controversial plan that gave a green light for Israel to annex large swaths of the occupied West Bank, including settlements considered illegal under international law, and the Jordan Valley.
Unveiled in late January, Trump's plan proposed the establishment of a demilitarised Palestinian state on the remaining patchwork of disjointed parts of the Palestinian territories without occupied East Jerusalem. The plan has been rejected in its entirety by the Palestinians.
The Security Council meeting, to be held by video conference, will be the last major international meeting on the issue before the July 1 deadline.
"Any decision on sovereignty will be made only by the Israeli government," Israel's UN envoy Danny Danon said in a statement on Tuesday.
Diplomats expect a vast majority of UN members to again oppose the Israeli plans on Wednesday.
"We must send a clear message," one envoy told AFP news agency, adding that it was "not enough" to simply condemn Israeli policy, and raising the possibility of a case before the International Court of Justice.
For decades, Israel has enjoyed bipartisan US support that enabled it to ignore international criticism and numerous UN resolutions over its occupation of Palestinian territories.
When Trump in late 2017 shifted US policy by recognising Jerusalem as Israel's capital, 14 of the 15 Security Council members adopted a resolution condemning the move - but the US used its veto.
A similar resolution was then presented in the UN General Assembly (UNGA), where no nation has veto power - it passed with 128 votes in favour, nine against and 35 abstentions.
Diplomats, however, seemed to rule out the idea that Israel could face sanctions over the planned annexation, as were imposed by certain countries after Russia's annexation of Crimea.
"Any annexation would have quite big consequences for the two-state solution in the peace process," another ambassador told AFP on condition of anonymity.
But the envoy said it was not a "simple task" to compare the West Bank with Crimea.
Earlier this month, hundreds of international law professors and scholars signed an open letter condemning the Israeli plans to annex territory in the West Bank, calling it a "flagrant violation of bedrock rules of international law, and would also pose a serious threat to international stability in a volatile region".
Kevin Jon Heller, an international law professor, told Al Jazeera the Israeli-planned annexation is a "clear and fundamental violation of international law that prohibits annexation of territories taken by force".
"Israeli annexation of the Golan Heights and Jerusalem coupled with international and Arab silence have emboldened it to take further action in that direction as its contemplating now," said Heller.
Source: The article was originally published by AlJazeera, along with the cover photo, captioned as: Guterres: 'This would be calamitous for Palestinians, Israelis and the region' [File: Reuters]