Six online gambling operators offering buy-in options on their slot games have now removed the features from their sites, after being contacted by the Gambling Commission.
The feature in question allowed customers to stake high sums to access the bonus feature immediately when playing, with one game charging more than £3,000 ($3,900) for the option.
The Gambling Commission said in a statement it is reminding operators to check their games meet the transparency and safety standards expected.
The statement also pointed out two requirements in the remote gambling and software technical standards but did not specify whether the feature broke either of the two rules.
RTS requirement 3A states: "An explanation of the applicable rules must be easily available to the customer before they commit to gamble.
"The content, including artwork and text, must be accurate and sufficient to explain all the applicable rules and how to participate. All reasonable steps must be taken to ensure the content is understandable."
RTS requirement 14A says: "Gambling products must not actively encourage customers to chase their losses, increase their stake or increase the amount they have decided to gamble, or continue to gamble after they have indicated they wish to stop."