Ukip leader Nigel Farage and others "hell bent on pulling the UK out" of the European Union would cause "enormous damage" to Britain's entrepreneurs if they got their way, Virgin boss Richard Branson has warned.
The entrepreneur's dramatic intervention comes as Farage prepares to debate tonight with Lib Dem leader Nick Clegg on Britain's place in the European Union.
Writing on his blog, Branson warned that the UK should not leave the European Union, even if the political bloc has a "few" faults.
"The last thing the UK should do is leave it, as it would then have no say on how to improve it and make it more productive for all countries involved.
"Leaving the EU would do enormous damage to entrepreneurs throughout the UK, both small and large. That in turn would inflict damage upon the rest of the country. It is entrepreneurs who create the jobs of the future; the last thing the government should want to do is make things more difficult for them by cutting ties with the EU.
"Fewer companies would set up business in the UK if they could only trade with 60 million people, rather than 500 million. If the UK shuts the door on the EU, the door will be firmly shut in return and it would be very difficult to get back into the EU once we had left."
Thomas Cook CEO Harriet Green recently warned against Britain leaving the EU and suggested that it could force the cost of holidays to rise. Other major businesses have come out including car giants Ford, Hyundai and consumer goods firm Unilever.
The Virgin boss compared the attitude of UK eurosceptics to the protests that broke out in Ukraine in support of closer ties with the European Union rather than Russia.
"For those people who seem hell bent on pulling the UK out of the EU, it is contrasting that the people of Ukraine were willing to die on the streets of Kiev in hope that they could join the EU," Branson wrote.
"There are countries all around Europe that are desperate to be a part of this great marketplace, and Britain should be desperate to remain a leading part of it."
Matthew Elliott, chief executive of Business for Britain, the Eurosceptic campaign backed by over 850 British business leaders, said:
"Richard Branson told Britain at the turn of the century that we'd be much worse off if we didn't join the euro. Now he is advocating Britain staying in the EU at all costs. The sensible option is not to defend the status quo or to leave tomorrow, but to push for real and substantial changes that will benefit all British businesses.
"The majority of UK entrepreneurs are totally in support of getting a better deal from the EU. That deal would include less regulation and a greater focus on trade and free trade deals, including with the likes of China, Russia and India with whom the EU is currently dragging its feet.
"No businesses want to lose access to the EU's 500 million consumers, but the vast majority also want to see real changes made to the terms of Britain's membership that will help them to grow and create more jobs."