Sir Richard Branson has warned that Brexit would be "devastating" for the long-term prosperity of the UK.
The billionaire founder of the Virgin Group empire penned an open letter as he joined the Remain campaign for a final push to persuade voters to keep Britain in the EU ahead of this week's referendum.
He recalled "how difficult it was" for businesses to operate effectively before the EU, adding he was "saddened" at the prospect of returning to those days.
A pro-Europe campaign will be spearheaded by the 65-year-old in the final stretch leading up to polling day - including his warning letter, adverts and a video message.
Virgin Group currently employs some 50,000 people in the UK and has had the British entrepreneur at its helm for more than half a century.
He said: "As an entrepreneur I have been known for taking risks throughout my career but leaving the European Union is not one of the risks I would want the UK to take - not as an investor, not as a father and not as a grandfather. I am deeply concerned about the impact of leaving.
"Although I've been living in the British Virgin Islands for some time now, I have never stopped caring passionately about the UK and its great people. I am one of the few business people who can remember how difficult it was before the EU was formed.
"Setting up a new business was a challenge, I couldn't move my employees between Britain and Europe without visas, moving goods was hugely complex with high taxes and the hassle of red tape and heavy-handed bureaucracy. I'm saddened to think the UK would ever go back to that time."
He added: "There are a few times in your life when you can see thing going horribly wrong. I truly believe that leaving the EU would be devastating for the long-term future of Great Britain and the future of Europe."
On top of presenting his economic case, Sir Richard also hailed the EU for securing peace on the continent in the wake of the Second World War.
He said: "My father Ted fought in North Africa, Italy and Germany during World War Two. My grandfather survived the horrors of the trenches in World War One.
"I truly believe that one of the EU's greatest achievements that it has kept its members out of conflict in Europe. In fact, not a single shot has ever been fired between the armies of EU member states."