An independent United Nations human rights expert has warned that plans to proceed with annexing significant parts of the occupied West Bank will create "a cascade of bad human rights consequences".
They are part of plans under the new Israeli coalition government, which will be led by Benjamin Netanyahu.
It was part of a pledge made by Mr Netanyahu if he was re-elected.
Michael Lynk is the Special Rapporteur on the situation of human rights in the Palestinian Territory.
In a new report, he said: "Israel's decision to unilaterally march ahead with the planned annexation on July 1 undermines human rights in the region, and would be a severe body blow to the rules-based international order".
It would also further undermine any remaining prospect for a just and negotiated settlement, he said.
"If Israel's annexation plans proceed, what would be left of the West Bank would become a Palestinian Bantustan, an archipelago of disconnected islands of territory, completely surrounded and divided up by Israel and unconnected to the outside world," he said.
"The plan would crystalize a 21st century apartheid, leaving in its wake the demise of the Palestinians' right to self-determination.
"Legally, morally, politically, this is entirely unacceptable."
And he said human rights violations arising from Israeli occupation would only intensify after the annexation.
"Already, we are witnessing forced evictions and displacement, land confiscation and alienation, settler violence, the appropriation of natural resources, and the imposition of a two-tiered system of unequal political, social and economic rights based on ethnicity."
Annexation has been strictly prohibited under international law since the adoption of the Charter of the United Nations in 1945.
Mr Lynk also expressed deep alarm that Israel's annexation plans are being supported and facilitated by the United States.
"On many fronts, the US was a positive force in the post-war years for the creation of our modern system of international law.
"It understood that a strong network of rights and responsibilities was the best path to global peace and prosperity.
"Now, it is actively endorsing, and participating in, a flagrant violation of international law.
"Its legal duty is to isolate perpetrators of human rights violations, not abet them."
Mr Lynk said the United Nations and its member states could no longer just offer criticism without consequences.
"The looming annexation is a political litmus test for the international community.
"This annexation will not be reversed through rebukes, nor will the 53-year-old occupation die of old age".
"There has to be a cost to the defiance of international law," he said, adding: "Only this can compel the Israeli political leadership to do the right thing."
The Tánaiste Simon Coveney has previously warned Israel against plans to annex parts of occupied Palestinian territory as part of any new government deal.
He said: "I think it important, as a friend and partner of Israel, to be very clear about the gravity of any such step.
"Annexation of territory by force is prohibited under international law, including the UN Charter, whenever and wherever it occurs, in Europe’s neighbourhood or globally.
"This is a fundamental principle in the relations of states and the rule of law in the modern world. No one state can set it aside at will."
He added that Ireland remains committed to a negotiated two-state solution that ends the occupation that began in 1967, with Jerusalem as the capital of both states.
This is in line with the European Union’s long-standing position.
"We firmly believe that this is the only outcome which will provide long term security, freedom and prosperity to both peoples", Mr Coveney added.
There are around 2.5 million Palestinians living in the West Bank, and it is seen by Palestinians as the heartland of any future state.
Meanwhile, there are also around 400,000 Israeli settlers living in the West Bank.
The settlements are considered illegal under international law - a view rejected by Israel.