Britain would be left "out in the cold" like North Korea if the country left the European Union (EU), according to Gordon Brown.
The former prime minister warned that the UK would have "few friends [and] no influence" if it left the economic and political union.
"We must tell the truth about the three million jobs, 25,000 companies, £200bn ($301bn, €280bn) of annual exports and £450bn of inward investment linked to Europe; and how the 'Britzerland' or Norwegian alternatives (even Norwegians oppose the Norwegian option) leave us subject to EU rules, but denied a vote in shaping them," he said.
"Being half-in half-out, a Britain that is semi-detached and disengaged, has made us weaker than ever. And we must talk about how the Hong Kong option – 'leaving Europe to join the world' – is really the North Korea option, out in the cold with few friends, no influence, little new trade and even less new investment."
The Scottish MP also stressed that pro-Europeans should champion reform of the EU and avoid "the trap" that those who want to stay in are for the status quo.
"It would be a terrible irony if Britain opts out, leaving Europe divided, Russia empowered, the US bypassing us for a Franco-German axis – and Scotland threatening to abandon a non-European UK," Brown wrote in The Guardian.
The comments come ahead of a House of Commons debate on the EU and after David Cameron promised to hold a referendum on the UK's membership of the union in 2017.
Labour and the Tories also face opposition from the right in the shape of Ukip, who oppose mass immigration and campaign on a Eurosceptic ticket.
The party gained the most seats at the EU parliamentary elections in the UK last year in and are consistently scoring around 15%.
Pollsters have predicted that Nigel Farage's party could win up to 10 seats at the general election in May.
The purple party currently has two MPs – Mark Reckless and Douglas Carswell – and hopes to get Farage elected in South Thanet.