This time there may be no easy way back."

Writing in the South China Morning Post, Brown said that despite major central banks drawing up contingency plans and getting ready to intervene if the UK votes to leave the UK, "it is hard to see what they can do." He added: "Global currency, credit and equity markets are already highly agitated."

Brown continued: "Europe should be especially worried. Whatever happens in this week's Brexit vote, Europe will feel deep shockwaves. It is a 'no win situation.' with Europe standing to lose whichever way the vote goes."

He noted that Europe is still "not long out" of the 2009-2013 eurozone crisis and it has taken huge stabilisation efforts by the EU governments and the European Central Bank to stop the European monetary union and the euro from "falling apart." Brown warned: "It would not take much to open up old wounds and set Europe lurching off into a new economic and financial crisis."