UK Prime Minister Boris Johnson has urged Israel not to go ahead with the annexation of part of the occupied West Bank, saying it would be illegal and "contrary" to the country's interests.
His Israeli counterpart, Benjamin Netanyahu, set Wednesday as a possible start date for the process of applying Israeli sovereignty to Jewish settlements and the Jordan Valley.
But that plan seems to have stalled.
The Palestinians say it will destroy their hopes of a viable future state.
The Israeli move would be in line with US President Donald Trump's Vision for Peace - a plan for ending the decades-old Israeli-Palestinian conflict that he unveiled in January.
Some 430,000 Jews live in more than 130 settlements (and scores of smaller "outposts") built since Israel occupied the West Bank in the 1967 Middle East war.
The settlements are widely considered illegal under international law, though Israel - and the US under the Trump administration - denies this.
Writing in the Israeli newspaper Yedioth Ahronoth on Wednesday, Mr Johnson described himself as "a passionate defender of Israel" but warned that annexation would represent a violation of international law.
"It would also be a gift to those who want to perpetuate the old stories about Israel."
The prime minister added: "I profoundly hope that annexation does not go ahead. If it does, the UK will not recognise any changes to the 1967 lines, except those agreed between both parties."
Mr Johnson said he was fearful that Mr Netanyahu's proposals would "fail in their objective of securing Israel's borders" and "be contrary to Israel's own long-term interests". They would also "put in jeopardy the progress that Israel has made in improving relationships with the Arab and Muslim world", he added.
The Palestinians want the West Bank as part of a future state and have rejected the Israeli proposals outright as a death blow to their hopes for self-determination.
The United Nations, the European Union and Arab countries have also called on Israel to scrap its annexation plans, saying they would violate international law, harm the prospect of a two-state solution, and undercut the possibilities of renewing the Middle East peace process.
The international criticism has so far not deterred Mr Netanyahu.
He has stressed that he is planning to extend Israeli sovereignty only to territory that would remain part of Israel under any realistic peace settlement, and that he will not include territory that the Trump plan has designated for a future Palestinian state.
However, there are signs of rifts within Israel's governing coalition over the timing of any annexation move, says the BBC's Yolande Knell in Jerusalem.
After meeting US envoys on Tuesday, Mr Netanyahu said talks would continue "in the coming days" and appeared to play down the importance of Wednesday's target date.
His former opponent-turned-coalition partner, Alternate Prime Minister Benny Gantz, also does not appear to be fully on board.
Asked if anything would happen on Wednesday, Mr Gantz replied simply that the sun would rise in the east and set in the west.
Earlier this week, he said dealing with the coronavirus pandemic and its socio-economic and health consequences was "the more pressing issue that needs to be tended to right now".