The United Nations, the European Union and key Arab countries have all said Israel's annexation would violate global law and undermine the already diminished prospects of establishing a viable independent Palestinian state alongside Israel. If it does, the United Kingdom will not recognise any changes to the 1967 lines, except those agreed between both parties.
"We can not support an undemocratic system in which Israel would permanently rule over a Palestinian people denied self-determination or equal rights", the letter, which has not been officially released yet, but a copy of which was made available to the media, read in part.
The step would be in line with a blueprint announced by Donald Trump in January under which Israel would control of 30 per cent of the West Bank. He proclaimed that the illegal Israeli West Bank settlements were not illegal.
Palestinians, who have rejected the Trump proposal, seek to establish a state in the West Bank and Gaza Strip, with East Jerusalem as its capital.
"So it is with sadness that I have followed the proposals to annex Palestinian territory", he added.
But the plan has come under stiff worldwide criticism.
Israeli media have suggested action could be postponed indefinitely, or that Netanyahu might announce a symbolic gesture to limit worldwide uproar, like the annexation of one Jewish settlement on the outskirts of Jerusalem.
The developments cast further uncertainty over whether Israel will ultimately follow through on the annexation initiative, which has drawn fierce worldwide condemnation from some of its closest allies.
While some settlers have urged Netanyahu to take similar action in the West Bank, other settlers oppose the Trump plan, as it envisions the creation of a Palestinian state across roughly 70 percent of the West Bank. The worldwide community considers the area occupied and its settlements to be illegal.
"Annexation would fundamentally breach a central prohibition under worldwide law of acquiring territory pursuant to the threat or use of force", they said. Legislature could be discussed from 1 July.
Annexation: July 1 has arrived, and the Israeli government's plan to annex part of the West Bank remains on pause.
But beyond the worldwide opposition, Netanyahu has encountered some resistance from his Blue and White governing partners. Israeli media also reported that Israel is seeking changes to a proposed US map for annexation, and that American officials are demanding an Israeli gesture to the Palestinians as compensation for any annexation that takes place.
"It seems unlikely to me that this will happen today", said Foreign Minister Gabi Ashkenazi, a member of the Blue and White party, which partnered with Netanyahu's Likud in an uneasy coalition government earlier this year.
USA officials said they would like the plan to have the support of Israel's two main parties, Netanyahu's Likud and Gantz's Blue and White.
Israel has postponed its controversial plan to annex portions of the West Bank, the country's regional cooperation minister said Wednesday, Anadolu reports.
Sen. Bernie Sanders, I-Vt., also signed the letter, saying in a statement that "American taxpayers shouldn't be supporting policies that undermine our values and interests, in Israel, in Palestine, or anywhere".