A British exit from the EU? There's still no plan for that

When Britain joined the European Union 42 years ago the commitment was to free trade.

Since then nine member states have become 28 and sovereignty has been shared.

At some point you'll be asked if this is club you want Britain to remain in.

Stuart Rose used to run Marks and Spencer.

He's a Tory peer and believes we'll be worse off if we leave.

Today his campaign came up with something eye-catching: a calculation that British households and businesses will pay an extra £11 billion a year in trade tariffs if we leave the single market.

Is he right?

Well, his underlying assumption is that Brexit will be a bitter affair, that import and export tariffs would be imposed by both Britain and the EU, damaging trade and therefore our prosperity. It's gloomy but plausible.

The problem is that Lord Rose's opponents argue the reverse. For them Britain's divorce with Europe would be a liberating, happy affair. A swiftly negotiated free trade deal would deliver ongoing access to the free market and greater independence.

In any assessment of the economic impact of Brexit we have to accept there are too many unknowns to establish definitive answers and that reasonable people reasonably disagree.

A group of economists from the LSE have tried to model the effect Brexit would have on trade.

Interestingly, they concluded that even in the best-case scenario Britain would face substantial costs.

The fact is if Britain leaves the EU a new relationship would have to be forged - one that would determine our future prosperity.

As it stands we've no idea what that relationship would look like.

The Prime Minister may threaten Brexit but his Chancellor has explicitly told civil servants NOT to prepare to it.

So there’s no timeline and no detail of the trade agreements the government thinks it could secure.

At the moment there is no plan B.