Adverts from online gaming operators Spin Palace and 888casino have been labelled misleading by the Advertising Standards Agency (ASA).
The regulator stated that both operators had not been clear enough when outlining wagering requirements for sign-up bonuses.
The 888casino ruling stemmed from a poster on the London Underground which promised "£88 Freeplay, no deposit needed".
However the ASA received two complaints that the poster did not mention the bonus had to be wagered 30 times before withdrawal.
The ASA ruling said: “We noted that the ad was untargeted and that a number of consumers might not be familiar with online betting or 888.com, or common practices such as wagering requirements.”
"We considered that the restrictions imposed regarding winnings and withdrawals was significant information which would inform an individual's understanding of the offer and their decision whether to sign up or not, and so should have been stated in the ad."
A similar edict was handed down to Spin Palace for a sign-up bonus advert on its website.
Text that gave information about wager requirements was "two clicks away" when it should have been on the same page as the ad, according to the ASA.
In both cases it was ruled that the adverts must not appear again in their current form