British Jews fear for their safety when they go to synagogues or the shops, according to the former Chief Rabbi, Lord Sacks.
Members of the nation's minority Jewish community are starting to wonder whether they need protection when they go out on normal daily tasks, he said.
Although he hoped it would dissipate quickly, Lord Sacks told BBC Radio 4's Today programme that the level of concern among British Jews was at the highest he had known in his lifetime.
He was speaking shortly after the Home Secretary Theresa May and Communities Secretary Eric Pickles took steps to reassure the Jewish community that the government is doing everything it can to fight antisemitism.
Speaking at #NousSommesJuifs, a Board of Deputies event to commemorate the lives lost in the Paris terrorist attacks, Mrs May said: "I know many Jews are feeling anxious. I never thought I would see the day that Jewish people would say they are fearful of remaining here in the UK – which means we must redouble our efforts to wipe out antisemitism in the UK."
She praised the contribution Jewish people make to the country but said the UK must make greater efforts to eradicate anti-Semitism.
"I never thought I would see the day when members of the Jewish community in the United Kingdom would say they were fearful of remaining here in the United Kingdom."
She referred to recent comments by the French prime minister Manuel Valls' who said: "If 100,000 Jews leave, France will no longer be French, the French Republic will be judged a failure."
Mrs May said: "It is a sentiment I well understand, one that holds true for Britain. Without its Jews, Britain would not be Britain, just as without its Muslims, Britain would not be Britain - without its Sikhs, Hindus, Christians and people of other faiths, Britain would not be Britain." she added.
Mr Pickles said that he stood "shoulder to shoulder with the Jewish community, shoulder to shoulder with fellow British citizens." He described antisemitism as "a cancer".
He said: "Modern Britain without a thriving Jewish community would not be Britain. The Jewish community is a vital part of what makes Britain tick, whether it is leadership through business and commerce, arts like dance or literature, or something more humble like members of the JLGB who give so much to the Jewish community and others."
The present Chief Rabbi, Ephraim Mirvis, said the community was deeply grateful to those who spoke up in support.
"While Britain is a tolerant and welcoming society, no place can consider itself immune, as Paris, Brussels and Toulouse have shown," he said. "As a community we appreciate the continued assurances and commitment of the government to fight antisemitism. We are fortunate here in the UK that the government and Parliament have been actively leading the fight against antisemitism. In the UK we have not needed a tragedy to trigger this concern."
According to a poll last week, nearly half of the British population holds an antisemitic view and Britain is at "tipping point" on the issue of antisemitism. The YouGov poll for the Campaign Against Antisemitism comes in the wake of of the Charlie Hebdo and other recent murders by Islamist fundamentalists, which included a deadly attack on a Jewish supermarket.
Police and other security has been stepped up outsie synagogues, schools and other Jewish sites.
Meanwhile, Labour leader Ed Miliband described himself as a "friend of Israel" after he was questioned about his support for the country following controversial comments during last summer's Gaza conflict.
The Jewish Chronicle reported that Mr Miliband attended a pre-election question-and-answer session with residents in Hendon, north-west London. Last summer he criticised Israel's operation in Gaza as "unacceptable and unjustifiable". He had also criticised David Cameron, suggesting the Prime Minister had been too timid in his response to Israel's actions.