The watchdog received 27,138 complaints in 2017 – down 4.8% on the previous year. But the number of ads complained about was up 14% to 19,398.
The internet overtook TV as the most complained about medium, scoring 10,932 complaints about 9,951 online ads, compared to TV’s 9,466 complaints about 4,666 ads.
The ASA said it had also resolved a record number of "own initiative" cases brought by its partner body the Committees of Advertising Practice – which it said was evidence of proactive action to protect consumers.
The figures appear in the ASA’s annual report, Showing More Impact.
The report outlines major regulatory interventions made by the ASA last year, which included ensuring alcohol ads were responsibly targeted online, ensuring speed claims by broadband providers were not misleading, and making sure influencer ads are labelled transparently.
Guy Parker, chief executive of the ASA, said: "We want to make sure ads are responsible without consumers necessarily having to complain to us. By being more proactive, we’ve secured the amendment or withdrawal of more ads than ever before.
"At the same time, we’ve delivered a record amount of advice and training to help businesses get their ads right before they run. Our approach is helping make ads more responsible, which is in the best interests of consumers, businesses and wider society."
Brands that saw their ads banned in 2017 included:
Despite receiving more than 500 complaints, KFC ad "The whole chicken" was not investigated.