Jake Marlowe, 26, went missing early on Saturday, the embassy said.
His mother, Lisa, told the Jewish News he was providing security at a party and she last heard from her son via text message.
"He was doing security at this rave and called me at 04:30 to say all these rockets were flying over," she said. "Then, at about 05:30, he texted to say, 'signal very bad, everything OK, will keep you updated I promise you,' and that he loves me."
Tzipi Hotovely, the Israeli ambassador to the United Kingdom, earlier told Sky News a British citizen was in Gaza. But she did not identify the name of the individual.
"I know that Israel is doing everything to make sure all the people being kept hostage at the moment in the hands of Hamas will be released," she said.
(Reporting by Andrew MacAskill, editing by Elizabeth Piper and Toby Chopra)
