Sir Ranulph Fiennes said leaving the European Union would be "utterly stupid and pathetic" after revealing he turned down an offer to campaign for Brexit.
The 71-year-old explorer, once named a Ukip patron, echoed Prime Minister David Cameron by saying a move to leave would be a "leap into the dark".
He said: "I got a letter from the Out people saying 'would I help them?' and I wrote back saying 'No, but if the In people want help I would try it'.
"I can think of nothing more utterly stupid and pathetic than leaving the European Union or Scotland leaving Britain. The EU is a vast market. OK, it's got things about it - everything's got wrong things about it - but for little England to step outside the market because they think things are going to get better - to leap into the dark - is utterly stupid."
One of the greatest adventurers in British history and once described by the Guinness Book of Records as "the world's greatest living explorer", Sir Ranulph does not believe voting out is the patriotic thing to do.
"We'll have a lot less power and a lot less influence and be 'little' Britain," he said.
"Therefore if you're nationalistic like me, you want to stick the Union Jack everywhere, you don't want to have the Scottish bit removed from the Union Jack, and you don't want us to be removed from a huge market."
The explorer was named a Ukip patron in 2004 but has since distanced himself over the issue of Britain's membership of the EU - despite still enjoying a long-standing friendship with the party's key financier Paul Sykes.
"I'm not a supporter of Ukip but there is a person who is involved with starting Ukip who helps us hugely with our charities.
"He is a wonderful helper but started Ukip whereas I desperately want to stay in the European Union, so we don't talk about that particular matter," he said.
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