"If it does, the United Kingdom will not recognize any changes to the 1967 lines, except those agreed between both parties", he said.
Israeli media have reported that Netanyahu is considering scaling back his plans and is expected to annex just a small number of settlements in a largely symbolic move.
With no announcement now scheduled on Israel's self-imposed kick-off date, opponents of the plan - notably Palestinians in the West Bank and Gaza - were mobilising.
A view shows the Israeli settlement of Ariel in the West Bank, on July 1, 2020.
The pro-Israel AIPAC lobby has lashed out at the four, which also includes Pramila Jayapal (D-WA) and Betty McCollum (D-MN4), saying the letter they wrote to Secretary of State Mike Pompeo "explicitly threatens the U.S. -Israel relationship in ways that would damage American interests, risk the security of Israel & make a two-state solution less likely".
The worldwide criticism has so far not deterred Mr Netanyahu.
Underlining that he was a "passionate defender" of Israel, Johnson said his country "has always stood by Israel and its right to live as any nation should be able to, in peace and security", he wrote, adding that the UK's "commitment to Israel's security will be unshakeable while I am Prime Minister".
Trump's proposal calls for Israeli sovereignty over about 30% of the West Bank - land on which Israel has been building settlements for decades - as well as creation of a Palestinian state under strict conditions. The protesters waved Palestinian flags and held signs calling the plan a "declaration of war" on the Palestinian people.
But the ethnically mixed community is also regarded as illegal by much of the worldwide community, as it is built on land gained by Israel during the 1967 Six Day War.
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The only way it can be achieved is for both sides to return to the negotiating table. He is eager to move forward before November's USA presidential election, especially with Trump's re-election prospects in question, and made sure that the coalition agreement for his new government included the July 1 date for him to introduce a plan to parliament.
Demonstrators in Gaza City took part in a rally on Wednesday as Palestinians called for a "day of rage" to protest Israel's plan to annex parts of the Israeli-occupied West Bank.
But beyond the worldwide opposition, Netanyahu has encountered some resistance from his Blue and White governing partners.
Friedman and Berkowitz are in Israel as part of the White House's efforts to win consensus within its government for annexation as envisioned in an Israeli-Palestinian peace plan announced by U.S. President Donald Trump in January.
Israel's defence minister and alternate prime minister Benny Gantz has said annexation must wait until the coronavirus crisis has been contained, amid a sharp spike in new Israeli and Palestinian cases. USA officials have said they don't want to move forward without both leaders in agreement.
The UN, EU and key Arab countries have said Israel's annexation would violate global law and undermine the prospects of a viable independent Palestinian state.
Brown is Israel, green Palestinian.
The majority of them, however, are pressuring Netanyahu to follow through, launching a campaign titled, "You made a promise - keep it".