Jeremy Corbyn has called on Theresa May to stand up for the rights of the Palestinian people when she meets Israeli Prime Minister Benjamin Netanyahu on Monday.
The Labour leader said that it was "simply not good enough" for Mrs May to say that Israeli settlement building on occupied Palestinian land undermined trust in the Middle East peace process.
He said Israeli government plans to build 3,000 new homes in the West Bank and East Jerusalem were illegal under international law and further undermined the prospect of a two-state solution to the conflict.
Downing Street has said that while Mrs May was expected to raise concerns about settlement building, it would only form a small part of their talks in No 10 which would focus on bilateral relations between the UK and Israel including the potential for more trade post-Brexit.
In a statement, Mr Corbyn, a long-standing campaigner for Palestinian rights, said: "That is simply not good enough.
"Fifty years after the United Nations demanded Israeli withdrawal from the territories occupied in the 1967 war and 70 years after the UN voted for the creation of a Palestinian state, the British Government must act in support of peace and justice in the Middle East conflict.
"Theresa May must make clear to the Israeli Prime Minister that the British Government will stand unequivocally behind the rights of the Palestinian people, along with the many who support them in Israel, as well as human rights and justice across the region."
He said the Prime Minister should also demand an assurance there would be no repeat of the "improper interference" by the Israeli embassy in London after an official was secretly filmed discussing how to "take down" Foreign Office Minister Sir Alan Duncan.