Labour's top donor has hit out at Gordon Brown's "hugely negative" claim that the UK would become the European equivalent of North Korea if it left the European Union (EU).
John Mills told IBTimes UK that the former prime minister's comments were "very extreme" and suggested the Scottish MP went "way over the top".
"One of the problems about people who want us to stay in the EU is that they tend to go way over the top and suggest that if Britain left the EU the world would come to an end. Really the situation is much more nuanced than that," the millionaire argued.
"There will be problems if we were to leave the EU and there would be some difficult negotiations that we would have to go through as well as a period of uncertainty.
"But there would be some advantages of us coming out as well. The idea that there's a sort of massive difference in the future – if we're in or out – is just not realistic."
Mills explained that he would want "sufficiently substantial" reform in Brussels for the UK to stay in the EU and claimed a majority of voters share his opinion.
"Most people in this country, given substantial changes, would want us to stay in the EU," he said.
But the entrepreneur warned that Labour could be "missing a trick" by not offering a referendum on the UK's membership of the EU as part of their election pledges.
"If Labour wants us to stay in the EU, which I think the majority of the party does, the best chance of settling the issue would be for the party to do what it can in terms of a re-negotiation of terms and then hold a referendum when it can put a positive picture forward," Mills said.
The major donor suggested that 2017 or 2018 would be the best year to hold the historical vote and warned that Labour could lose in 2020 if they party does not offer it.
"The danger is if it doesn't do this the country will get more and more Euroscepitc, then you will have a right-of-centre government elected in 2020 on a Brexit platform," he said.
The self-described "dyed-in-the-wool" Labour supporter gave the party £1.65m ($2.47m, €2.32m) in shares of his TV shopping company in 2013.
However, Mills revealed that Miliband's position on the EU would not dissuade him from giving the party more money.
"I'm a dyed-in-the-wool Labour supporter. Any democratic party has people who disagree with bits of what it says, I'm still a strong supporter. I wouldn't have differences like that," he said.
Mills made the remarks ahead of the 16 March launch of his new book, Call to Action.
The businessman explained that pamphlet attacks the "cross-party consensus on austerity" and proposes a range of "radical changes" to economic policy.
YouGov, with 55 days to go before the general election, put Labour one point ahead of the Tories (35% vs 34%) in the polls.
The survey of more than 1,700 voters also put Ukip on 14%, the Liberal Democrats on 7% and the Greens on 5%.