Israel intends to annex West Bank settlements and the Jordan Valley, as proposed by US President Donald Trump, with initial steps slated to begin from July 1.
Following a virtual meeting of OIC foreign ministers, the pan-Islamic body said it "warns against the dangerous intention of Israel".
The annexation threat amounts to "an official declaration by Israel of revocation of all its signed agreements" and "a serious escalation of its colonial policies and measures", the OIC said in a statement.
Read also: German FM warns Israel over West Bank annexation plans
Saudi Arabia's Foreign Minister Prince Faisal bin Farhan reiterated the move would represent a "dangerous escalation" that threatens the chances of resuming the peace process.
Israeli annexation forms part of the US peace plan Trump unveiled in January, which paves the way for the eventual creation of a Palestinian state.
The proposals exclude core Palestinian demands such as a capital in east Jerusalem and have been rejected by the Palestinian Authority.
Palestinians have sent a counter-proposal for the creation of a "sovereign Palestinian state, independent and demilitarised" to the Quartet, made up of the UN, US, EU and Russia, according to Palestinian prime minister Mohammed Shtayyeh.
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