Piratin was elected at the 1945 General Election as Member of Parliament (MP) for Mile End in Stepney, becoming one of the last two CPGB MPs. In Parliament, he worked with several left-wing Labour MPs, some of whom would be expelled by their party as crypto-communists and form the Labour Independent Group. He was defeated when he stood for re-election in 1950 in the new constituency of Stepney - his old seat of Mile End having been abolished due to boundary changes.