In March 1942, he arrived in London to join the National Council of the Polish government in exile, where he was one of two Jewish members (the other was Ignacy Schwarzbart). In London, Zygielbojm continued to speak publicly about the fate of Polish Jews, including a meeting of the British Labour Party and a speech broadcast on BBC Radio on June 2, 1942. On June 25, 1942, "The Telegraph" reported the existence of Nazi gas chambers and the mass killing of Jews, based on Szmul Zygielbojm's information.