"While there has been interest from potential sponsors, no one has come anywhere near the asking price and there is concern that further delay could lead to the one offer being withdrawn.
"That would mean the championship lacking a backer in 2018 and that would blow a significant hole in the finances of the three Celtic unions, Wales, Scotland and Ireland."
The paper said the failure of media outlets to mention RBS in written reports about the Six Nations was another factor to overcome.
The Six Nations chief executive John Feehan was in a buoyant mood prior to this year's tournament, stating they had the "biggest sponsorship in world rugby by a country mile", saying they had made contact with more than 150 companies.
Feehan talked about seeking a sponsor "who's actively engaged not just in Britain but around the world and see potential in markets like the USA."
The Guardian wrote: "...with the financial services sector no longer a major player because of tighter regulation and global brands not convinced about the value of a championship played in western Europe, tournament executives have found it a tough sell."