The Prime Minister declared “terrorism will not prevail” and pledged “steadfast support” for his counterpart Benjamin Netanyahu as the death toll from the incursion climbed to at least 600 on Sunday.
It comes as the family of 20-year-old Nathanel Young, a Briton serving with the Israeli Defence Services, said he had been killed on the Gaza border on Saturday.
Two other UK nationals – 26-year-old Jake Marlowe, who went to the same London school as Mr Young and photographer Dan Darlington – are also thought to be missing.
In a statement delivered from Chequers, the Prime Minister said: “The scenes that we’ve seen in Israel over the past 36 hours are truly horrifying.
“I want to express my absolute solidarity for the people of Israel now is not a time for equivocation, and I’m unequivocal. Hamas, and the people who support Hamas, are fully responsible for this appalling act of terror, for the murder of civilians and for the kidnapping of innocent people, including children.”
Mr Sunak said he had spoken with the Israeli PM earlier on Sunday to offer the UK’s ongoing support, adding: “Terrorism will not prevail.”
Asked what assistance is being provided to British citizens caught up in the conflict, he told reporters: “This is a dangerous and fast-moving situation on the ground. And I know that there will be families who are anxious about their loved ones.
“I want to reassure them that the Foreign Office and the Government here is in close contact with our Israeli counterpart to establish the status of any British nationals on the ground.”
Mr Young, who was a former pupil at JFS Jewish School in north London, is among hundreds to have died following a surprise attack by Palestinian militants which caught Israel off-guard on a major holiday.
Gaby Shalev, from London, posted on Facebook: “We’re heartbroken to share that our little brother Nathanel Young was tragically killed on the Gaza Border yesterday.”
Labour leader Sir Keir Starmer said his thoughts were with Mr Young’s family, condemning the “terrible violence” which he labelled a “terrorist attack”.
“This is a grave and significant moment,” he said. “The indiscriminate attacks from Hamas are unjustifiable and have set back the cause of peace.”
The Board of Deputies of British Jews warned that previous conflicts in the Middle East have seen an increase in antisemitism in the UK and said it was in touch with Government to ensure the Jewish community is supported.
The organisation is also holding a vigil near Downing Street in central London on Monday for those killed or injured in the violence.
On Sunday evening, the Israeli flag was projected on to the outside of No 10 in a show of support for the country.
Mr Marlowe was reported missing while providing security at a music festival near Kibbutz Re’im, close to the Gaza border.
Mr Darlington, who was visiting from his home in Berlin, Germany, is also missing, according to his father David, as reported by the BBC.
Mr Young’s brother, Eliot Young, said in a statement released to the BBC: “Nathanel was full of life and the life of the party. He loved his family and friends and was loved by everyone.
“He loved music and was a talented DJ, DJing both on base and at Benji’s House, the lone-soldier house where he was living.”
Mr Young had been living in the Bayit Shel Benji lone-soldier house in Raanana, according to the Jewish News.
The Israeli Embassy in London has confirmed his death to the PA news agency and the Foreign, Commonwealth and Development Office (FCDO) has been approached for comment.
The FCDO has advised against all but essential travel to Israel and Occupied Palestinian Territories due to the conflict.
The Israeli Embassy in London confirmed that London-born Mr Marlowe, who is reported to have moved to Israel two years ago was missing, but it had not been confirmed that he had been kidnapped.
A spokesperson told PA: “He is missing, we don’t know for sure that he is taken hostage or dead or in a hospital.”
Mr Marlowe’s mother Lisa, told the Jewish News: “He was doing security at this rave yesterday and called me at 4.30am to say all these rockets were flying over.
“Then, at about 5.30am, he texted to say ‘signal very bad, everything OK, will keep you updated I promise you’, and that he loves me.”
Israeli media have reported that at least 600 people were killed, including 26 soldiers, during the attacks, prompting Mr Netanyahu to say they were at “war” as the country launched a military response.
Foreign Secretary James Cleverly said he had spoken with his Israeli counterpart Eli Cohen on Sunday and made clear the UK is “resolute” in its support for Israel “in the face of terrorism”.
Israeli ambassador to the United Kingdom Tzipi Hotovely confirmed that a British citizen “is in Gaza” following kidnappings by Hamas, but did not clarify if they were being held hostage.
She told Sunday Morning With Trevor Phillips on Sky News: “I know there is one British citizen who is in Gaza at the moment.
“The Israeli government is doing everything we can to help those who are held hostage and every citizen who is taken is returned.”
Foreign Affairs Committee chairwoman Alicia Kearns said the Government’s priority would be ensuring the safety of the missing Briton and as well as other British nationals in Israel.
She told Sky News: “What we did see yesterday was British families having to hide in safe rooms some with, I believe, a baby as young as 10 days old, and so many international victims being taken hostage.
“Obviously, that will be a priority for the British Government, I would expect the ministers will be in today working on precisely how we can get that individual to safety working with our allies.”
Ms Kearns placed the blame for the attack on Iran for supporting Hamas and called for the proscription of Iran’s Islamic Revolutionary Guard Corps (IRGC) which she described as the country’s “international terror tool”.
A Foreign Office spokeswoman said they would not comment on individual cases but added: “However, we can confirm we are in contact with, and assisting, the families of several individuals in Israel and the OPTs (Occupied Palestinian Territories).
“The safety of all British nationals continues to be our utmost priority and we urge everyone to continue to follow our travel advice which is updated regularly.”