The defeat of the British government in a referendum on EU membership would certainly be a seismic event. There is nothing comparable to it in British political history. It would have profound consequences for our politics, for our economy and for our position in the world. And it could have consequences that would be constitutional rather than merely political.
In our new report, we at British Influence believe the first few days the focus would be on the immediate political consequences. Would the prime minister be able to stay in office? Would the government fall? How would the markets react? What should the government and parliament do about pending EU business?
Soon after the future of the Union would become a major question. As we say in the report, "all the available evidence indicates that the risk of a vote to leave triggering a renewed Scottish independence campaign is high". This is partly because opinion polls suggest that Scots are more likely to vote for the UK to stay in the EU than English voters, raising the possibility of the UK as a whole voting to leave while Scotland votes to stay in. But it is also because Nicola Sturgeon, Scotland's First Minister, has said that such a situation would be a trigger for the holding of a second referendum on independence.
And then there is the Irish dimension. The fact that we are both members of the EU has normalised the once sour relations between the UK and the republic. The open border between north and south has reduced the sense of isolation felt by some people in the nationalist community and facilitated commerce. A British exit from the EU could result in new border controls between the two and a weakening of the republic's economy.
The importance of the EU to food, farming and fisheries is well known but establishing new markets for our food - the UK's largest manufacturing export - securing access to the fishing grounds of other EU countries and devising a new support mechanism for our farmers would all be necessary and challenging.
Many of those arguing for Britain to leave have made it all sound so deceptively simple: one vote and we're out. It wouldn't be like that, even in the most benign scenario. Leaving would require us to confront some fundamental questions about our own country and its economic and foreign policies. Above all, the central economic question of how to retain easy access to the single market from outside the EU. Our report lays down the gauntlet by identifying the top 10 questions that supporters of Brexit need to answer.
Those who argue for exit need to put forward a credible alternative, one that offers the UK the same advantages for trade, jobs, economic growth, our environment and our security as EU membership. We challenge them to do so.