The UK government said on Thursday that the country was facing an antisemitism emergency and pledged to increase security for Jewish communities after a string of arson attacks and a double stabbing that have sparked fear and anger among Jews.

The country’s official terror threat level was raised from substantial to severe after Wednesday’s stabbing attack in London, which police have called an act of terrorism with potential links to Iran. Severe is the second-highest rung on a five-point scale...