These more attractive territories include Spain, where under “Beckham’s Law”, named after the famous footballer, gains on non-Spanish assets are not taxed. Ireland also has its own non-dom regime allowing foreign gains and income to grow tax-free.

Sir Martin Sorrell, the chairman of S4 Capital and founder of advertising giant WPP, warned that with “increased mobility in the digital age” there could be “a considerable exodus and avoidance” in the event of a capital gains crackdown.

Small business owners are scrambling to sell their companies owing to mounting fears that Ms Reeves will increase CGT in the Budget on Oct 30, wealth advisers said.

Industry leaders warned that it will be hard to win back wealth creators who abandon Britain, adding that the country would risk missing out on thousands of new jobs as a result.

Charlie Mullins, the founder of Pimlico Plumbers who has recently relocated to Spain, said: “I don’t like the idea of the capital gains [changes], I don’t like the idea of inheritance tax.

“Any property I have in the UK under my name I will be selling. I still have a place there now in Westminster, but that will be getting sold.

“I know quite a few millionaires and billionaires who have left the UK, set up in Monaco or Dubai. Italy are offering a good deal now.

“I know a lot of people have moved their money from the UK. Not just because of tax, but because of Labour’s policies on workers’ rights, and on most things.”

The billionaire Tory donor Lord Spencer, a City tycoon who gave £250,000 to Rishi Sunak’s election campaign this year, said that Labour was hoping to “turn lead into gold” as there is “no example of a high tax, high spend and high growth economy” anywhere else.

Fears of a tax raid among Britons who own businesses, property and other assets have intensified over the summer after the Chancellor announced a £22bn hole in the public finances.

Gains made from selling a business are currently taxed at 20pc but many investors are concerned that Labour will seek to equalise these rates with income tax, which is 45pc for additional rate payers.

This means many business owners who sell up  after the tax changes would lose a significant amount to the Treasury.