The joint press conference was spurred by common opposition against US President Donald Trump's controversial peace plan, which paves the way for Israel to annex territory in the occupied West Bank, including Jewish settlements considered illegal under global law.
Organized by the Office of the Chief Justice of Palestine in partnership with the Palestinian Ministry of Waqf and Religious Affairs, the conference comes as a response to Israel's Prime Minister, Benjamin Netanyahu's U.S. backed-pledge to annex the West Bank beginning July 1.
Palestinian Prime Minister Mohammad Shtayyeh, speaking at a June 9 briefing with reporters, called annexation "an issue in which we can not be silent".
Johnson, in an op-ed piece that appeared in the Israeli daily Yedioth Ahronoth, asserted that annexation threatens budding ties between Israel and the Arab world.
There were smaller demonstrations in the West Bank cities of Ramallah and Jericho, which some left-wing Israeli politicians attended. The global community considers the territory to be occupied and Israel's more than 120 settlements to be illegal.
There has been no major announcement on Israel's self-imposed kick-off date, but Mr Netanyahu said he was also discussing annexation with his security chiefs, saying "further discussions will be held in the coming days".
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Johnson, who described himself in the opinion piece as a "passionate defender of Israel", said the United Kingdom would not recognise any changes to the pre-1967 borders in the West Bank that were not agreed to by both Israelis and Palestinians.
"Annexation is an existential threat for our future", he said.
"As a life-long friend, admirer and supporter of Israel", he continued, "I am fearful that these proposals will fail in their objective of securing Israel's borders and will be contrary to Israel's own long-term interests".
But the plan to redraw the Mideast map has come under fierce global criticism.
In the statement, the Vatican quotes the Pope's speech from that June 2014 meeting where Francis called on Israel and Palestine to have "the courage to say yes to encounter, and not to conflict: yes to dialogue and no to violence; yes to negotiations and no to hostilities; yes to respect to agreements and no to acts of provocation; yes to sincerity and no to duplicity". Netanyahu, a close ally of Trump, has said Israel must take advantage of what he calls a "historic opportunity", and is eager to move forward before the November U.S. presidential election. Israel now has full control over 60% of the West Bank. The Palestinians have rejected the plan.
There has even been domestic opposition in Israel with the country's defence minister and alternate Prime Minister, Benny Gantz, who has said annexation must wait until the coronavirus crisis has been contained.