However, Mr Johnson – despite describing the UK as a “strong supporter of the ICC” – has strongly opposed the inquiry in a letter to the Conservative Friends of Israel.
“We 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,” the prime minister has written.
“This investigation gives the impression of being a partial and prejudicial attack on a friend and ally of the UK’s”.
Nevertheless, Mr Johnson insisted the UK had an “enduring commitment to strengthening the court” in The Hague and in “serving international justice”
Richard Burden, the former chair of the Britain-Palestine all-party parliamentary group, criticised the prime minister for thinking he “knows the jurisdiction” of the ICC better than the court itself.
He tweeted: “He says UK opposes this investigation of a ‘friend and ally’. The UK either supports independence of the court or it does not. We can’t have it both ways.”
The ICC had made clear its investigation “will be conducted independently, impartially and objectively, ‘without fear or favour’,” Mr Burden pointed out.
Mr Johnson’s stance comes despite him sharply criticising, last year, Israel’s plans to illegally annex much of the West Bank, warning they would deny “justice” to Palestinians.
The long-awaited inquiry – announced last month – followed an earlier ruling that the court’s jurisdiction extends to territories occupied by Israel since the 1967 six-day war.
Human rights groups were delighted, but, in a furious response, Israel branded the ICC “morally and legally bankrupt”.
Mr Netanyahu, the Israeli prime minister, said: “The decision of the international court to open an investigation against Israel today for war crimes is absurd. It’s undiluted antisemitism and the height of hypocrisy.”
Joe Biden’s administration has also said it disagrees with the ICC taking the action, but the US – unlike the UK – is not a party to the court.
The ICC’s chief prosecutor, Fatou Bensouda, said it would look into “crimes within the jurisdiction of the court that are alleged to have been committed” since 13 June 2014.
The investigation would be conducted “independently, impartially and objectively, without fear or favour,” the Gambian lawyer added.
If it identifies suspects allegedly responsible for crimes, prosecutors can ask judges to issue international arrest warrants.
Hazem Qassem, a Hamas spokesperson in Gaza, said: “We welcome the ICC decision to investigate Israeli occupation war crimes against our people. It is a step forward on the path of achieving justice for the victims of our people.”