Lord Sugar has compared Britain exiting the European Union with The Great British Bake Off leaving the BBC - warning both will be a "disaster".
The Apprentice host, who sits in the House of Lords and was in May appointed enterprise tsar by David Cameron, warned foreign investors will flee the country in the wake of Brexit.
Lord Sugar, who has been a vocal critic of Britain leaving the EU, made the comments at the London press launch of the latest series of The Apprentice.
Asked about Brexit, he said: "I fear for our country. I was very clear in my views. I think they have made a fatal mistake.
"A lot of people are saying things don't seem bad, when we had the referendum, and nothing seems to have changed. Well exactly, nothing has changed because we haven't even started yet.
"I forecast the biggest disaster in five years' time as, for example, as Bake Off is going to be on Channel 4, and as a comparison - when you get round to doing all these rules and regulations and changing all that we need to do to trade with European countries, we are going to see a complete change in our country."
He added: "I fear for large organisations, big foreign organisations will move out and go to other European capitals."
The billionaire businessman also laid into Republican presidential candidate Donald Trump, declaring the prospect of the mogul in the White House "a frightening thought".
Mr Trump fronts the American version of The Apprentice and the two men have a long time rivalry, frequently having spats over Twitter.
Reflecting on the first presidential debate, Lord Sugar said: "What comes to mind is, it was like a US version of Would I Lie To You?
"It's quite a frightening thought that he may become one of the most powerful men in the world shortly."