Seventeen British nationals, including children, are dead or missing after the Hamas attack on Israel.
It is an increase on the previous estimate of “more than 10”.
The death of Israelis has reached 1,200, with more than 900 people killed by Israeli air strikes on Gaza.
Foreign Secretary James Cleverly has travelled to Israel, with the Foreign Office saying the visit was to meet survivors and outline UK support.
A Foreign Office spokesperson said Mr Cleverly was in Israel “to demonstrate the UK’s unwavering solidarity with the Israeli people following Hamas’ terrorist attacks”.
In additional: “He will be meeting survivors of the attacks and senior Israeli leaders to outline UK support for Israel’s right to defend itself.”
The deaths of Nathanel Young, Bernard Cowan and Jake Marlowe have been confirmed.
Mr Young was a 20-year-old who attended JFS School, a Jewish school in North London, and was serving in the military in Israel.
The school’s headteacher, David Moody, said the school’s community was “devastated” and “heartbroken” at the news of his death.
He added: “Nathanel is fondly remembered within the school and we think of him with nothing but love.”
Mr Young’s funeral, held at Israel’s national cemetery Mount Herzl, was interrupted after loud bangs were heard over Jerusalem.
More than 1,000 people turned out and listened as Mr Young’s younger brother Elliot paid tribute to him.
But when his sister started to remember him, an emergency siren pierced the tranquillity and prompted mourners to throw themselves to the ground, taking cover under trees and between gravestones.
Bernard Cowan grew up in Glasgow before settling in Israel with his wife and three children.
His family said in a statement: “We are grieving the loss of our son and brother, Bernard Cowan, who was horrifically murdered on Saturday during the surprise terrorist attack on Israel by Hamas.
“We ask for privacy at this time while we process this huge loss to our family, both at home and in Israel, and to the Jewish community in Glasgow where he will be sorely missed.”
Jake Marlowe was working as a security guard at the Supernova music festival, where 260 people were killed when it was stormed by militants. On Wednesday afternoon, the Israeli Embassy in London confirmed he was also among the victims.
The 26-year-old has been reported as missing after the attack, which took place at the Re’im kibbutz around 3.7 miles (6km) from the Gaza barrier.
Mr Marlowe was also a former pupil of JFS in North London.
The family of Daniel Darlington have also said they believe he is among those killed.
In a statement posted on Instagram, his sister referred to him as Danny and her “baby brother”. She said he was killed at the Nir Oz kibbutz alongside a friend.
He said he had spoken to ministers in the Israeli government.
“We have asked the Israeli government to let us know what they need. And again, we continue to talk to them about that.”
Shadow foreign secretary David Lammy has written to Mr Cleverly to ask what steps are being taken to ensure that British people who want to leave Israel are able to do so.
Mr Lammy, whose Tottenham constituency is home to a significant Jewish population, told the foreign secretary that while Israeli airspace “has not officially been closed”, most UK airlines have been “forced to cancel their flights for the foreseeable future”.
“This is obviously deeply concerning for all those who are desperate to return to loved ones in the UK,” he said.