This would, in effect, at least double the rate paid on the sale of most high-income assets.
Ms Rayner voiced her criticism after it emerged that Prime Minister Rishi Sunak paid 22 percent in taxes last year, as a significant portion of his income comes from capital gains.
UK wages in excess of £50,270 are taxed at 40 per cent, rising to 45 per cent. for those earning more than £125,140.
Profits from high earners who sell a second home or rental property are taxed at 28 percent, while the sale of other assets such as stocks, a business and personal belongings such as jewelry are taxed at 20 percent.
A survey by start-up consultancy Beauhurst previously found that 85 percent of founders would consider moving their business abroad if the tax hike went through.
While concerns about a change of government were the most cited reason in the Evelyn Partners survey, entrepreneurs planning to sell also cited a number of other factors.
Challenges in accessing long-term capital or investments to keep their business viable was the second most chosen reason, also cited by one in four owners planning to sell.
A similar percentage said changes in their personal circumstances meant they had to access cash tied up in their businesses.