Caledonian Trust Plc, the Edinburgh-based property investment holding and development company, said in its interim results that the economic prospects for an independent Scotland "are not favourable".
Chairman Ian Douglas Lowe said he expects the status quo to prevail but advised the "politically brilliant" Alex Salmond has the potential to lead a close run campaign.
"A week is a long time in politics and the referendum is not for 26 weeks," said Mr Lowe.
"The money is on No at 1/3 but as I write I have just heard that Lord Windermere, a 20/1 outsider, having trailed the field at Cheltenham's Gold Cup, put on a magnificent finish to win by a nose.
"The politically brilliant Mr Salmond possesses what the FT describes as an almost 'Gandalf-like ability to turn the political tables on opponents'.
"Historically, this 'Lord of the Rings' reversed a large deficit before the 2011 polls election to gain the SNP an absolute majority in Holyrood."
Mr Lowe warned that the costs of independence will be "very high" for businesses, particularly Scotland's integral financial sector.
A formal currency union is "extremely unlikely", joining the euro may be the "least bad thing to do" but a separate currency pegged to the pound may be Scotland's "only realistic option", he said.
He added: "The only choice entirely within Scotland's discretion is to have a fully independent currency - the Scots Crown, the Pound Scots or, John Kay's choice, the Bawbee first minted by the father of Mary, Queen of Scots, surely a dramatic statement of true independence with a hint of romanticism.
"The currency options possess one common factor - they will all come at a cost.
"A substantial and real attraction of the independence vote is based on idealism, romanticism, perceived past injury or slight, nationalism, kinship and social responsibility and the vaunted egalitarian tradition in Scotland. All these are powerful forces, many of them of admirable merit.
"It is right that Scotland makes a choice on principle but it will be an expensive club to join with a continuing high subscription.
"I suspect that the status quo will prevail, but if so it will be a much closer-run thing than is currently perceived. In conclusion I forecast only marginal damage to Scotland's economic prospects."
Several companies have included the referendum in the "risk management" sections of their investor reports as September 18 approaches.
On Friday, Edinburgh-based investors F&C Private Equity Trust and Baillie Gifford said respectively that independence threatens the "consistency, stability, familiarity and economies of scale" it enjoys as part of the UK, and creates "political uncertainty which may have practical consequences".
Dan Macdonald, chief executive of Edinburgh-based property investor Macdonald Estates, said: "My main concern, as both a businessman and a Scot, is to get the truth from a Westminster Government about where our economic prospects lie.
"After the last election, George Osborne said he would reduce national debt to zero from that which he inherited from Brown and Darling. In fact, by the end of his term it will be £1.5 trillion.
''Westminster simply isn't working for Scotland. Independence will equip the Scottish Parliament with the financial tools it needs to tailor policy and priorities to Scotland's particular needs, to secure investment and stimulate growth in the economy, and to create jobs."