Financial firms continue to flout rules designed to protect investors from being misled about the true value of financial products, according to a recent bulletin from the U.K.’s Financial Conduct Authority (FCA).

The regulator warned firms that practices such as “flying” and “printing” create “a false impression of a financial instrument’s liquidity and/or price.” Not only does this unethical behavior damage reputations and undermine trust in the market, but it might also mean firms are breaching the U.K.’s Market Abuse Regulation and the Financial Services Act, as well as sections of the FCA’s handbook.

Ruth Prickett graduated from Cambridge University with a BA hons in History and has specialized in business and finance journalism for the past 20 years. She was editor of Financial Management, the magazine...