Nathanel Young is the only named British victim, but there are fears for the safety of others.
More than 10 British citizens are feared dead or missing in Israel after the weekend’s attack by Hamas, an official UK source has told the BBC.
Nathanel Young, who was serving with the Israel Defense Forces, is the only Briton confirmed to have died.
But the BBC understands photographer Dan Darlington is thought by his family to be dead.
More than 700 people have been killed in Israel since Hamas launched its attacks on Saturday morning.
That includes 260 at a music festival where Briton Jack Marlowe was providing security. He has been reported missing.
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Bernard Cohen, from Glasgow, has also been identified by family members on social media as having been killed in the attack.
The north London school attended by Mr Young, who was 20, is “devastated” by his death, its headteacher has said.
He went to the same school – North London’s JFS Jewish School – as 26-year-old Mr Marlowe.
Mr Marlowe was reported missing from the Supernova music festival near Kibbutz Re’im, close to the Gaza border, which came under attack from Hamas militants.
JFS headteacher Dr David Moody said: “We are heartbroken at what has happened.
“Nathanel is fondly remembered within the school and we think of him with nothing but love. Equally, we have seen reports that Jake Marlowe is missing and we all pray that he is found soon.”
He continued: “As a school of 2,000 pupils we have stood together in prayer this morning and I know that the thoughts of every staff member and every student are with both families at this horrific time.”
Mr Young’s funeral is being held in Israel later.
The UK government believes around 50,000 to 60,000 British nationals are in either Israel or Gaza.
It has said it is assisting the families of several individuals in Israel and in the Israeli-occupied West Bank and Gaza Strip.
The Foreign Office has advised against all but essential travel to the region due to the conflict.
Prime Minister Rishi Sunakhas heldan emergency Cobra meeting later after days of escalating violence.
Mr Sunak said the UK government was working “very closely” with its Israeli counterparts to “establish the status of any British nationals on the ground”.
He called the scenes from Israel “horrifying” and said “Hamas and the people who support Hamas are fully responsible for appalling acts of terrorism – call it out for what it is”.
The government has no plans for a UK-facilitated evacuation of British citizens from Israel, with those in need of support advised to contact the Foreign Office.
Following the Cobra meeting Foreign Secretary James Cleverly refused to speculate about the number of Britons involved but said it was “unprecedented”.
He said that due to Israel’s “unique status” there were a large number of British-Israeli dual nationals – many of whom planned to stay in Israel or who were members of the Israel Defense Forces.
“We have made consular numbers available for any British nationals who are seeking to leave. Flights are still available so there are opportunities to leave Israel if people want to do that,” he said.
“And of course, as the situation develops we will remain in very, very close contact with the Israeli government to make sure that we’re providing the support that they need to defend themselves against this terrorist attack.”
Scotland’s First minister Humza Yousaf has said his parents-in-law are “trapped” in Gaza.
He said the Israelis have told them to leave but they have no way to get out and the UK Foreign Office cannot guarantee safe passage.