 In August 1903, as a member of the Main Administration of the SDKPiL, he was one of two Polish delegates to the Second Congress of the Russian Social Democratic Labour Party (RSDLP) in Brussels.The Congress later adjourned to London, under pressure from the Belgian police, and there the RSDLP split into its Bolshevik and Menshevik factions, but Hanecki and the other Polish delegate, Adolf Warski, did not make the journey to London, having failed to agree terms on which the RSDLP and SDKPiL could collaborate.At the outbreak of the 1905 revolution, Hanecki returned illegally to Warsaw, with Felix Dzerzhinsky to run the SDKPiL's underground organisation, until Jogiches arrived, in October.