Britain would become the North Korea of Europe if it severed ties with Brussels, Gordon Brown has suggested.
Quitting the European Union would leave the UK "out in the cold with few friends" and "no influence", the former prime minister said.
Mr Brown likened a Britain outside the EU to dictator Kim Jong Un's pariah state in an article for the Guardian.
He wrote: "We must tell the truth about the three million jobs, 25,000 companies, £200bn of annual exports and the £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.
"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."
Prime Minister David Cameron has promised the British people a vote on whether to pull out of the EU by the end of 2017 if he remains in No 10 after the general election.
Labour has said it will only hold a referendum if there is a proposal to transfer further powers to Brussels.
Winning over support for the EU means setting out the how the "true patriotic" course is for Britain to lead in Europe, Mr Brown said.
He wrote: " It would be sheer defeatism to cast ourselves, as sceptics do, as helpless victim, impotent bystanders unable to influence events.
"Our destiny is not a bit player on someone else's stage, or a spectator hectoring from the wings, but always setting the agenda, bringing people together, and championing change."
He added: "Being half-in half-out, a Britain that is semi-detached and disengaged - the Britain of the empty chair even when we are in the room - has made us weaker than ever: irrelevant on Greece, fringe player on climate change, mere spectator in the debate that could have shaped a European pro-growth policy, marginal on Ukraine, with ministers sounding ludicrous as simultaneously they say: 'Russia must be confronted with a more united Europe', and: 'By the way, we are thinking of leaving.'"