Harold Wilson, the then Prime Minister, made it quite clear when reporting to Parliament on the UK application to join the EEC that it was made for political and economic reasons.

“But whatever the economic arguments, the House will realise that, as I have repeatedly made clear, the Government’s purpose derives, above all, from our recognition that Europe is now faced with the opportunity of a great move forward in political unity and that we can — and indeed must — play our full part in it.” – Harold Wilson to the House of Commons 2nd May 1967 from Hansard.