David Haigh, chief executive of Brand Finance, which describes itself as the world's leading brand valuation consultancy, said uncertainty surrounding the result of the referendum was "already hitting investor confidence".
He said the break-up of Britain would not only affect the brand value of England but also an independent Scotland, suggesting neither would have the high brand profile Britain currently had.
The consultancy said that, according to its research, Brand Britain had never been more valuable and was now worth, following growth of 8%, £1.4trillion. Britain is now fourth in the brand value global league table, pushing Japan into fifth place.
As Prime Minister David Cameron sells Brand Britain in China, Brand Finance made reference to the UK Government's current campaign to sell the nation abroad called "GREAT Britain".
But it claimed the "brand equity" this campaign had built up was under threat from the 2014 poll.
The consultancy pointed out how a Yes vote would mean the end of Britain as a political entity, "forcing a profound reappraisal of national identity and rebranding of the two independent nations. Millions in nation brand value could be lost with tangible economic consequences".
Mr Haigh said: "Scottish independence, however noble the motives of its supporters, represents a significant threat to both Scotland and England. The nation brand value built up over centuries and in the last few years so successfully fostered by the Government's GREAT Britain campaign would be squandered.
"The combined nation brand values for an independent Scotland and England are likely to be substantially lower than that of a united UK; in the medium term at the very least.
"Brand Finance estimates strong nation branding can add between 1% and 5% to GNP (gross national product). In the current economic environment, no sensible government can afford to ignore branding as an instrument of economic policy," he added.
A spokesman for Better Together, a pro-Union campaign group, said: "Scotland has an incredible brand that is known around the world, but we also benefit enormously from being part of the United Kingdom.
"We don't need to choose between the two. We have the benefit of being distinctly Scottish and the benefit of being part of the UK. We have the best of both worlds."
But a spokesman for pro- independence group Yes Scotland said: "Brand Scotland has had a powerful global resonance for hundreds of years from its hills and glens, water to whisky, scholars to shipbuilders, authors to artists, industrialists to innovators.
"Any suggestion that a Yes vote would mean starting afresh is clearly nonsense and not a considered opinion worthy of consideration or concern."
He added: "What would be fresh, however, is the impetus and confidence being independent would bring to our nation's wide spectrum of talent in all fields and to enhance a brand that is trusted worldwide."
Brand Finance said it used analysis more usually applied to companies to draw up its annual Nation Brands report, which showed the impact a country's reputation and image had on governments, investors and consumers.
A combination of government statistics, forecasts, and analysis is used to quantify a number of variables to create an overall brand rating.
According to its league table for 2013 the top places for brand value were: USA: $17,990billion, up $3349bn on 2012; China: $6109bn, up $1263bn; Germany: $4002bn, up $99bn; UK: $2354bn; up $165bn and Japan - $2263bn, down $289bn.
The country in the bottom 100th place was Albania with a brand value of $8bn, down $2bn on the previous year.