Ed Miliband can perhaps be forgiven for his reluctance [2] to leap in and commit a Labour government to holding an in/out referendum on Britain’s membership of the EU. After all, in a speech to the CBI last November he warned [3] that fellow EU countries are "deeply concerned" because they sense Britain is "heading to the departure lounge". But realpolitik now makes it matter of when and how he follows David Cameron’s lead in calling for a referendum, not if.

When that moment comes, he could do worse than to learn a thing or two from his predecessor-but-seven, Harold Wilson. As prime minister, Wilson did what Cameron now promises to do: renegotiate the terms of Britain’s membership and then hold a referendum on the outcome. Like Cameron today, his referendum was about fudging internal party divisions as he took a seemingly outrageous chance with the country’s future strategic interests. But Wilson – who won more general elections (four) than any other 20^th century prime minister – was no slouch when it came to campaigning and his approach to the 1975 referendum bears a number of useful lessons for Miliband now.

1. A referendum can be cathartic. In his 1976 book The Governance of Britain, Wilson wrote that the referendum "ended once and for all the previous doubts about the support of the British people for membership of the European Community." It’s a high-wire act, but the potential reward for winning a referendum on Europe could be to bring to an end to decades of sniping and foot-dragging about Europe. And it would set up Ed as a leader prepared to lead boldly.

2. Hold a vote quickly. Wilson’s famous dictum that "a week is a long time in politics is especially insightful when it comes to holding referendums. It is wise to move quickly before other problem sget in the way and the vote becomes a stick to beat a mid-term government. The 1975 referendum was held just nine months after the October 1974 General Election on 6 June.

3. A win is a win: don’t set a threshold for support or turnout. The 1975 referendum was passed with 67 per cent support on a 65 per cent turnout. However Labour’s disastrous decision just four years later to impose a condition that 40 per cent of Scotland's total registered electorate had to vote in favour during the referendum on a Scottish parliamentsaw the vote fall short (just 52 per cent voted yes on a 64 per cent turnout). This precipitated the collapse of Jim Callaghan’s Labour government.

4. Public opinion will shift. Just as now, a majority of people in 1975 were sceptical about European membership at the start of the referendum campaign. But as Peter Kellner points out [4], now, as in 1975, opinion "is not completely fixed"; with the renegotiated terms providing a potentially useful new element which can be used to undermine existing hostility.

5. Labour is more united now than it was then. A referendum is often a time for friends to agree to disagree. Wilson suspended the cabinet’s collective responsibility courtesy of an "agreement to differ". He had huge political figures like Michael Foot, Tony Benn, Peter Shore and Barbara Castle ranged against him and much of the trade union movement hostile to Europe. Miliband is far more fortunate; divisions within the Labour tribe about Europe are puny compared to those confronting Wilson in the 1970s. Miliband’s problem is not about getting colleagues to vote yes, but how to keep them aligned as he choreographs a change of policy to now support a referendum.

6. A referendum forces the political centre ground to co-operate. Wilson, Roy Jenkins, Denis Healey and Jim Callaghan – the big beasts of the Labour government – were united in favour of a 'yes' vote in 1975. As were much of the Conservative and Liberal parties. Indeed, one opposition figure was particularly forthright: "If we fail and the British people vote ‘no’ to the European Community, they will read how there was a defeat for co-operation between nations…They will read how extremism won over commonsense."

So said [5] Margaret Thatcher.