|From its November 1975 launch until its immersion into Labour Leave in January 2016, the Labour Common Market Safeguards Committee was the primary Eurosceptic grouping within the Labour Party. It has hitherto received scant attention in the historiography even though, until 1983, it was one of the most influential groups within the party, included Cabinet Ministers amongst its membership and played a significant role in Labour adopting a policy of EEC withdrawal in 1980. After 1983, Safeguards’ standing in the party declined precipitously as Labour moved towards a pro-EEC standpoint, leading historians to dismiss them as amateurish and in hibernation. Nonetheless, throughout the following three decades, its members produced bulletins and pamphlets expounding the case against European integration from a Labour standpoint. Studying this intellectual case highlights that, although in recent years Euroscepticism is predominantly found on the right of British politics, it cannot be fully understood unless its history within the Labour Party is also considered. This history is far more influential than commonly believed. Moreover, historians have tended to consider Euroscepticism within Labour as limited to the hard left wing of the party – yet Safeguards eschewed the hard left, instead cleaving to an older version of Labour encapsulated by Attlee and Gaitskell. Their arguments proved highly effective in the 2016 referendum, as Safeguards’ members belatedly embraced modern campaigning technologies to reach a wider audience. Their role in the campaign sheds new light on why around 37% of Labour voters supported Leave, without which Brexit would not have happened. Yet this success was four decades in the making.