The UK could lose a staggering £250bn ($366bn) worth of trade a year if it broke away from the EU, according to Labour grandee Lord Alistair Darling. The Britain Stronger in Europe spokesman and former chancellor issued the warning to coincide with the group's new analysis of EU Free Trade Agreements (FTAs).

The group, which is chaired by former Marks and Spencer boss Lord Stuart Rose, estimated that the UK's trade to EU FTA nations accounts for 60% or £673bn of Britain's total trade every year. This trade is £250bn a year higher than it would have been if the UK was not a member of the 28-nation-bloc, according to Stronger In.

"Those wanting to leave the EU want to pull Britain out of the single market, which would mean introducing tariffs and barriers to our trade and putting billions of vital trade at risk," Darling argued.

"The choice is between free trade within the EU's single market of 500 million consumers, or spending years negotiating new trade deals only to leave us in a weaker position than we enjoy today.

"Leaving the single market would be catastrophic for our businesses and our families who would be paying more and suffering from a weaker economy.

Stronger In also claimed that, on average has taken the EU six years to negotiate an FTA, suggesting that it could be years before the UK has its own FTA after a Brexit.

But the Vote Leave campaign stressed that the UK is the world's fifth largest economy and accused the pro-EU group of undermining Britain's financial importance.

"[Stronger In] can't even be consistent or honest in their campaign to do down the British economy," said Matthew Elliott, the chief executive of Vote Leave.

"Their underlying belief appears to be that Britain -- the world's fifth largest economy and a nation with a great history of trading across the globe -- would be an economic backwater if it wasn't for Brussels taking control of our trade deals. That's absurd.

"After we Vote Leave we will take back control of the powers we've surrendered to EU bureaucrats and stop sending Brussels £350m a week. That would boost our economy and allow us to spend our money on our priorities."

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