The changes, announced in December and enforced from Friday onward, ban companies from using depictions of gender "that are likely to cause harm, or serious or widespread offense."
Broadcast, online and print advertising is affected by the guidelines, which will force advertisers to discard dated and stereotypical portrayals of men and women.
Advertisers will have to tread carefully in scenarios the watchdog cites as problematic. These include commercials that show a man with his feet up while a woman cleans; a man or woman failing at a task because of their gender; suggestions that a person's physique has held them back from romantic or social success; or a man being belittled for performing stereotypically "female" tasks.
Examples of potentially problematic material given by the Advertising Standards Authority (ASA) include a man failing to change a nappy or a woman struggling to park a car; a commercial suggesting that mums should prioritize cleanliness or looking attractive over their emotional wellbeing; and ads that play on stereotypical ideas of gender, such as men being more "daring" or women being more "caring."
"Our evidence shows how harmful gender stereotypes in ads can contribute to inequality in society, with costs for all of us," Guy Parker, Chief Executive of the Advertising Standards Authority, said in a statement Friday.
"Put simply, we found that some portrayals in ads can, over time, play a part in limiting people's potential. It's in the interests of women and men, our economy and society that advertisers steer clear of these outdated portrayals, and we're pleased with how the industry has already begun to respond."
The rules will be applied on a case-by-case basis, with each commercial individually assessed to decide whether the code has been broken.
The ad -- for a weight-loss product -- was not banned by the ASA, as it did not explicitly break any rules, but the regulator eventually banned it for its health claims, ASA spokesman Craig Jones told CNN.
While some on social media dismissed the new code as too politically correct, Jones called on critics to look at the evidence of "real-world harms of people not living up to their full potential" due to many factors, including sexual stereotypes in advertising.
The ASA said commercials will still be allowed to show "glamorous, attractive, successful, aspirational or healthy people or lifestyles."
Ads that show men or women carrying out tasks with which their genders are sometimes associated, such as women doing the shopping or men doing DIY tasks, will also be permitted.
Commercials featuring just men or just women are also unaffected, as are those that show gender stereotypes "as a means to challenge their negative effects."