The 1975 referendum took place under a Labour government and both major parties were in favour of continuing with our membership.
The 2016 referendum took place under a Conservative government and both major parties were in favour of continuing with our membership. It was only because of a disconnect between the government and the public that support for UKIP rose to such a level that Cameron felt threatened and offered a referendum. Both major parties were still in favour of continuing with our membership.

In more recent years it seems difficult to tell whether the party leaders (particularly May, Corbyn & Starmer) support membership of the EU or not.