In a statement, the FCA said that over a period of nearly two years, the individual, who was employed at a bank, took into account positions on their own trading book when making benchmark submissions, tried to influence other bankers' submissions and colluded with a trader at another bank, submitting quotes at his request.
The warning is the third to be issued in as many weeks - the first time the FCA has issued warnings since it was given the power to do so.
In the past, the financial watchdog would only make its enforcement proceedings public once an individual or company was punished.
The FCA's ability to issue early warnings has raised concerns that a firm or individual would be permanently tarnished even if a case were subsequently dropped.
The FCA is likely to take many months before it reaches final decisions on the two individuals, who can challenge them in an independent FCA committee and later in court.
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