Marks & Spencer's former boss says a Brexit will lose every British business £600,000
Sir Stuart revealed the figure on the BBC's Today Programme and laid out the basic maths, based on figures from the Center for European Reform.
Sir Stuart said that there are £345 billion of goods that are traded between Britain and the EU and, of that amount, £135 billion is attributable to the EU. If you divide that figure between the 200,000 odd business in the UK, you get of loss of somewhere between £600,000 and £700,000.
Brits are set to decide whether the UK should remain in the European Union in a referendum before 2017 that could come as soon as this summer.
Credit Suisse recently released a note saying that a Brexit could hit UK GDP by 2 percentage points - a significant blow.
The obvious objection to Sir Stuart's figures from someone who wants Britain to leave the EU is that that the UK will quickly negotiate a new trade agreement with EU as soon as it leaves.
The Chief Executive of the Vote Leave campaign, Matthew Elliott, responded to Sir Stuart's claim by calling his figures "cloud cuckoo land economics" and pointing out that the Center for European Reform, where Sir Stuart sourced his figures from, has also said the UK would secure a free-trade deal with the EU if it votes to leave it.
Elliott says:
This is yet more cloud cuckoo land economics from the BSE campaign. Its claims ignore the cost of the EU to our economy, our democracy and our wallets. As the CER admits, after we Vote Leave the UK would secure a free trade deal with the EU allowing it to trade with the whole of Europe without constantly handing money and power to Brussels.
The problem with Sir Stuart's £700,000 claim and the counter-argument that the UK would secure a new trade deal with the EU is that we simply have no idea what a type of negotiation the UK could secure.
Meanwhile, while both sides argue about theoretical numbers, immigration is the biggest issue for voters and the referendum vote might rest on whether Prime Minister David Cameron manages to get other EU leaders to agree that the UK shouldn't have to pay in work benefits to new EU migrants.