Prestige Financial Markets, owned by AllProTech OÜ, is believed by the regulator to be providing financial services or products in the UK without its authorisation.
The firm is based in Tallinn, Estonia and has another address in Zurich, Switzerland.
An FCA spokesman said: “Almost all firms and individuals offering, promoting or selling financial services or products in the UK have to be authorised by us.
“However, some firms act without our authorisation and some knowingly run investment scams.
“This firm is not authorised by us and is targeting people in the UK.
“Based upon information we hold, we believe it is carrying on regulated activities which require authorisation.”
The regulator warned consumers to only deal with authorised financial firms.
The alert was the second regarding an Estonia-based company this week, after it released a similar warning about GoldTech Media Services OU, trading as Glenmore Investments, which is also based in the Baltic country.