London (CNN Business) Japan's richest man has warned that Brexit is "practically impossible" and could take the United Kingdom back to the economic stagnation of the 1970s when the country was often described as the "sick man of Europe."

Fast Retailing FRCOF Tadashi Yanai, the billionaire founder and CEO of, said the United Kingdom had benefited for many years from being an open economy. Brexit would change that , he said, and would prompt many talented people to look for employment elsewhere.

Leaving the European Union also presents huge challenges because of the need to preserve an open border between Ireland (an EU member state) and Northern Ireland (part of the United Kingdom), and because of calls for independence in Scotland, which voted by a clear margin against Brexit in the 2016 referendum.

"I think Brexit is practically impossible because the old borders will be shaky and the UK has a Northern Ireland issue and a Scotland issue," Yanai told CNN Business. "Therefore, I think Brexit is difficult to realize even if the UK wants to do it."

"If Brexit does happen, the UK could revert back to the former situation before the Margaret Thatcher era, when the UK was referred to as the sick man of Europe. I'm afraid that could happen again."

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