Former Labour chancellor Alistair Darling has told talkRADIO that leaving the European Union would be a 'massive unquantifiable risk' for the United Kingdom.
Darling and his successor as chancellor, Conservative George Osborne, have joined forces to write an open letter criticising the Leave campaign, saying their proposals are "uncosted and unworkable" and that political rivals such as Boris Johnson and Michael Gove are "making it up as [they] go along".
Darling explained why he believes the UK needs to stay in the EU.
"Our economy is growing, but everybody knows that growth is fragile," he told Julia Hartley-Brewer.
"Why on earth would you want to take on a massive unquantifiable risk that you'd be knocked for six if you chose to leave?
"We would take a hit - a hit in terms of jobs, [and] in terms of the amount of money any future chancellor would have in terms of spending on health and education.
"One of the reasons we are one of the biggest economies of the world is because we are also part of the biggest single market, the European Union."
The former Labour MP, who served in the Cabinet for 13 years under Tony Blair and Gordon Brown, claims figures released by organisations such as the Bank of England and Treasury illustrate the dangers of a leave vote.
"Whatever way you look at these figures, whether you take one side or the other, they all point the same way," he said. "Even some of the Leave people have said 'yes we'll take a hit'.
"It's important that those who want to leave the EU, the Leave campaign, tell us what would happen.
"They have got alternatives, but they will never come clean about which one."
Darling also voiced strong concerns over Britain's future trade arrangements with mainland Europe, stressing that the right to free movement remains a key cornerstone of entry into the single market trading bloc.
"Working in one country can only get you so far," he said. "There are advantages of being in the European Union, principally economic, but also social, which I do not think we should throw away.
"Both France and Germany have made it clear that if we're going to get another trade agreement we would have to accept the free movement of people, which is the one thing the Leave campaign say they don't want."