Always a passionately committed supporter of European integration, Williams was one of 68 Labour MPs to defy a three-line whip in the 28 October 1971 Commons vote on membership of the European Communities. Four years later, she was one of the leaders of the Britain in Europe campaign during the 1975 European Communities membership referendum. Labour's anti-Europeanism during the Michael Foot years was one of the factors that drove her to abandon the party in 1981.