The City watchdog said the firm branding itself as Sarasin Funds Management (Guernsey) Limited was believed to be targeting UK investors.
But it is not authorised to carry out any regulated activities in the UK, which include providing advice and dealing or arranging investment deals.
The FSA stressed the bogus firm does not have any association with a list of FSA authorised firms that have Sarasin in their company name.
These are: Sarasin & Partners LLP; Sarasin Asset Management Ltd; Sarasin Investment Funds Ltd; Sarasin Funds Management (Guernsey) Ltd; Sarasin Funds Management (Ireland) Ltd; Sarasin Investmentfonds SICAV and Bank Sarasin AG.
Additional details of the firms and their FSA authorisation numbers are available on the FSA website.
Investors should be aware, the FSA warned, that the Financial Ombudsman Service and Financial Services Compensation Scheme would not be available if money was lost through dealing with an unauthorised company or individual.
It added that investors and advisers could check whether a company was authorised by looking at the FSA Register Check Service.
An up-to-date list of unauthorised firms is also available.