Timothy Garton Ash is a historian, political writer and Guardian columnist.

As we approach the 40th anniversary of Britain joining what was then just the European Economic Community, there is only one good way forward for the tortuous domestic politics of Britain's so-called European policy. This is for the leaders of the three main parties in the Westminster parliament to commit themselves to hold a straightforward "in or out" referendum once the shape of the new European Union that is emerging from the eurozone crisis, and the terms available in it for Britain, have become clear.

Since the eurozone is now likely to be saved, but only quite slowly, step-by-step, à la Merkel, and since Britain's position can only be clarified once the political consequences of saving the eurozone have emerged, that moment will arrive some time in the life of the next parliament: between 2015 and 2020, on current plans.

This is what David Cameron's tantrically delayed Europe speech, which he has now scheduled for mid-January, should promise. If Ed Miliband and Nick Clegg have the guts and gumption, they will beat Cameron to it and steal his thunder – not to mention, some of Ukip's lightning. All of them can quite reasonably refer to the exhaustive review of the "balance of competences" between the UK and the EU being conducted across Whitehall, and to be completed only in 2014, as a starting point for the conversation across the Channel. There would then be a settled national position. We, the people, will have the chance to decide whether we want to be in or out, as soon as we have an answer to the essential prior question: "In or out of what?"..