The PM said the state had an “absolute right to defend itself from the barbarity of Hamas” and offered counterpart Benjamin Netanyahu unconditional support.
His intervention comes as Israel declared Palestinian terrorists had “unlocked the gates of hell” after a bloody rampage left at least 700 innocents dead.
The world continues to hold its breath as the Holy Land promised to “use all its strength” to destroy the militants behind the bloody atrocity.
Mr Sunak said: “I spoke with Prime Minister Netanyahu to assure him of the UK’s steadfast support as Israel defends itself against these appalling attacks. We will do everything we can to help. Terrorism will not prevail.”
He added: “As the barbarity of the atrocities becomes clearer, we stand unequivocally with Israel. This attack by Hamas is cowardly and depraved. We have expressed our full solidarity.”
The Islamic Resistance Movement launched an audacious assault from Gaza, infiltrating the south of Israel, before embarking on an indiscriminate killing, kidnap and torture spree.
It was the largest and bloodiest attack for 50-years and sparked immediate global condemnation.
Israeli Prime Minister Benjamin Netanyahu reiterated the warning, adding: “We will be victorious in this war despite an unbearable price. What happened is unprecedented – and I will see to it that it does not happen again.
“All of the places which Hamas is deployed, hiding and operating in, that wicked city, we will turn them into rubble.
“I say to the residents of Gaza: Leave now because we will operate forcefully everywhere.”
Nathanel Young, 20, a serving Israeli soldier became the first confirmed British victim of the worst violence to grip the Middle East in decades, after Palestinian militants launched cross-border dawn raids on Saturday.
Fellow UK national Jake Marlowe, 26, who went to the same Jewish Free School in London as Cpl Young and moved to Israel in 2021, is missing, feared dead.
The world recoiled in horror as images showed Hamas soldiers bundling bloodied and shackled hostages, including women, children and soldiers, into trucks, before driving them across the border.
It is thought terrorists have captured 100 Israelis, although the real figure is thought to be significantly higher, hugely complicating retaliatory strikes and military operations to free them.
They are being held as human shields with fundamentalists expected to use them as a bargaining chip for the release of more than 4,000 terrorists currently being held in Israel.
It is now almost certain Mr Netanyahu will order a mass ground assault on Gaza, during which hundreds are likely to be slaughtered.
His promise to wipe the Sunni-Islamic group, founded in 1987, from the face of the earth has plunged the powder keg region into chaos, sparked worldwide panic and raised the spectre of nuclear war.
Iran-backed militia Hezbollah, the Lebanon-based Shia terrorist group formed in 1982 in response to the Israeli invasion, is poised to attack the north of the country and has already fired a volley of rockets into Israel.
Israel is widely believed to possess an arsenal of nuclear weapons with estimates putting its stockpile between 80 and 400 warheads, although the state has never publicly acknowledged their existence.
However, it has repeatedly stated it “will not be the first country to introduce nuclear weapons to the Middle East” and is not a signatory to the Treaty on the Non-Proliferation of Nuclear Weapons, despite international pressure, saying it would be contrary to national security.
They could be delivered by submarine-launched cruise missiles, aircraft or as ground-based ballistic missiles.
Israel has internationally-recognised sovereignty legally inherited from Britain in 1948.
But Palestinians claim territory – encompassing the Gaza Strip and West Bank, including East Jerusalem – has been illegally occupied since 1967.
Since 1994 Hamas has stated it would accept a truce if Israel withdraws to the 1967 borders, paid reparations, allowed free elections in the territories and gave Palestinian refugees the right to return.
Mr Netanyahu promised to wreak vengeance for the invasion, turning Gaza “into rubble”, after publicly declaring war with Hamas, which governs Gaza, after soldiers launched a surprise attack early on Saturday morning during Shabbat, the Jewish day of rest, after thousands of rockets were fired from the Gaza Strip.
Hundreds of gunmen burst into southern Israel, killing soldiers and civilians, and taking into Gaza a “significant number” of hostages.
In what was a massive security breach and huge humiliation for Israel, heavily-armed militants stormed the south of the country during the Jewish Sabbath and festival of Simchat Torah.
Gunmen slipped in through perimeter fencing, storming into Israel on motorbikes, paragliders and in a series of amphibious assaults.
Lt Col Jonathan Conricus, of the Israel Defence Forces, said: “They executed Israeli civilians in cold blood in their homes and then continued to drag into Gaza Israeli civilians and military personnel. I’m talking about women, children, elderly, and the disabled.”
Israel responded with a volley of air strikes, and promised full-scale revenge. Britain, America, the EU, Japan, and Australia have all designated Hamas a terrorist organisation, although not the UN.
It is feared the latest campaign of terror was backed and funded by Iran. It has left at least 2,000 injured.
The US was poised to move navy ships and military aircraft closer to Israel in a show of strength and support, officials said.
US President Joe Biden said: “Let me say this as clearly as I can.
This is not a moment for any party hostile to Israel to exploit these attacks to seek advantage. “My support for Israel’s security is rock solid and unwavering.”
Australia, Belgium, France, Germany, India, Ukraine, Canada, Poland, and Sweden also issued public declarations of support.
Mr Netanyahu said: “Hamas invaded Israeli territory and murdered innocent citizens including children and the elderly. Hamas has started a brutal and evil war.
“Hamas wants to murder us all. This is an enemy that murders children and mothers in their homes, in their beds, an enemy that abducts the elderly, children and young women, that slaughters and massacres our citizens, including children, who simply went out to enjoy the holiday.
We will destroy them and we will forcefully avenge this dark day that they have forced on the State of Israel and its citizens. As Bialik wrote: ‘Revenge for the blood of a little child has yet been devised by Satan’.
“All of the places which Hamas is deployed, hiding and operating in, that wicked city, we will turn them into rubble.
“I appeal to all citizens of Israel: We stand together in this campaign. This war will take time.
It will be difficult. Challenging days are ahead of us. However, I can promise one thing: With the help of God, the forces that we all have in common and our faith in the Eternal One of Israel, we will win.”
