David Cameron told 1,200 guests at Jewish Care's annual dinner that the charity is "the best of Jewish and the best of British".
But Steven Lewis, Jewish Care chairman, said that the government's decision to raise the minimum wage meant the charity needed an extra £4 million every year.
During a week in which swastikas were discovered at a children's playground in Stamford Hill, the prime minister promised that he would protect the community from antisemitic attacks. Speaking on Monday at the Grosvenor House Hotel in central London at an event which raised £5.1m for the charity, Mr Cameron told guests: "I care passionately about your security.
"Sadly we've seen growing examples of antisemitism here at home. I promise that wherever we see antisemitism, from politicians who should know better to abusive graffiti daubed on synagogues and homes, I will do everything I can to drive it out of the country."
The prime minister received standing ovations from the crowd, which included Chief Rabbi Ephraim Mirvis and Israeli ambassador Mark Regev.
The loudest applause came when he said: "Many things in my diary have been moving around, but there was one engagement I was always going to keep, and that was speaking at this dinner.
"I love Jewish Care and what you do. You don't just leave things to the government; you make a difference, you help others, you bring people together to play their part, and that is the big society: Jewish Care in action."
Mr Lewis outlined plans for the charity, including a campus for care on the Sinclair House site in Redbridge and a new independent living site in Hendon.
"Donors may ask, why do them all now? Why by 2020?" he said. "That year is significant because by then the government is intending for the national minimum wage to be above £9 per hour, and this is going to have a dramatic impact on our cost base. By 2020, we will need an additional £4 million each year just to survive."
Leona Lewis, the former X Factor winner entertained the crowd with songs including her biggest hit, Bleeding Love.
