Britain is against an International Criminal Court investigation into alleged war crimes in the Palestinian territories, Prime Minister Boris Johnson said in a letter published on Wednesday.
The United States had already criticised the inquiry, which was launched last month after the ICC ruled it has jurisdiction to examine possible crimes committed since 2014.
“We oppose the ICC’s investigation into war crimes in Palestine,” Mr Johnson said in the letter to three politicians from his right-wing Conservative party, which was originally sent on April 9.
"We do not accept that the ICC has jurisdiction in this instance, given that Israel is not a party to the Statute of Rome and Palestine is not a sovereign state," said Mr Johnson, whose office confirmed the letter's authenticity to The National on Wednesday.
The court is set to examine alleged crimes committed by Palestinians and Israelis in East Jerusalem, Gaza and the West Bank.
Palestine joined the ICC in 2015 and accuses Israel of “ongoing, systematic, and widespread” crimes.
"This is a long-awaited step that serves Palestine's tireless pursuit of justice and accountability," the Palestinian Foreign Ministry said in March, when prosecutor Fatou Bensouda opened the investigation.
In addition to investigating the 2014 Gaza war, the ICC will also examine long-standing Israeli policies, including settlement building in the West Bank.
Israel says it does not recognise the court's jurisdiction and Prime Minister Benjamin Netanyahu described the investigation as absurd.
“We will never stop fighting this injustice. We will speak the truth in every forum, in every country, on every stage until this outrageous decision is reversed and becomes null and void,” Mr Netanyahu said last month.
Israel has the backing of US officials, who “disagree strongly” with the ICC’s actions, Secretary of State Antony Blinken said.
"We maintain our long-standing objection to the court's efforts to assert jurisdiction over personnel of non-states parties such as the United States and Israel," he said on April 2, as the US dropped sanctions imposed on ICC officials for a separate inquiry into possible US war crimes in Afghanistan.
Israel has controlled East Jerusalem and large parts of the West Bank since the 1967 war. It also seized Gaza before pulling out of the enclave in 2005 and imposing a blockade on the coastal territory.