Hannan, a committed pro-European, was one of 69 Labour MPs to break a three-line whip and vote for Britain's entry into the European Economic Community in October 1971. He was, however, opposed to Scottish devolution, and was involved with the 'Scotland is British' campaign prior to the 1979 referendum on Home Rule. Always on the right wing of the Labour Party, in 1981 he became a founder member of the breakaway Social Democratic Party (SDP), and in February of that year was one of 100 signatories to an advertisement in "The Guardian" supporting the Limehouse Declaration.