David Cameron has pledged to help Belgium tackle “bigotry across Europe” after a gunman killed three and left another in critical condition at the Brussels Jewish Museum.
Officials said it bore the hallmarks of an anti-Semitic attack.
“I was appalled to hear of the shootings at the Jewish Museum yesterday,” Mr Cameron told him.
“We join you in condemning this dreadful attack and will work with you to confront such bigotry across Europe.
“Our thoughts are with the families and friends of the victims.”
Belgian prosecutors are hunting a lone suspect.
An Israeli couple and a French national were killed while a Belgian is in a “very critical condition”.
Foreign Secretary William Hague also expressed shock and sadness in a letter to his Belgian counterpart Didier Reynders.
“Tragic events like these only strengthen our resolve to work with you and others to eliminate such shocking, extremist, discriminatory behaviour in Europe and elsewhere across the world,” he said.