An ad by campaigning group Fathers4Justice accusing the Mumsnet website of labelling men as rapists, paedophiles and wife beaters has been banned by the advertising watchdog.

Fathers4Justice ran a national press ad in March this year with the headline "Say it with hate this Mother's Day", alongside a picture of a toddler with various negative words written all over his body including pig, rioter, wife beater and rapist.

The ad, addressed to companies that advertise on Mumsnet, claimed that the online parents' forum carried abusive and distressing anti-male content which promoted gender hatred against men and boys.

"We believe that the general sexist labelling of men and boys as 'rapists', 'paedophiles' and 'wife beaters' is as unacceptable and offensive as racism and homophobia," the ad said, calling on the website to adopt a zero tolerance policy to gender hatred.

The Advertising Standards Authority received 10 complaints that the ad made misleading claims which could not be substantiated.

Fathers4Justice remained unbowed by the accusations, arguing that the advert actually underplayed the seriousness and gravity of the content on the website.

The group supplied a number of screenshots of activity on Mumsnet to back up the claim, and said it believed that Mumsnet was responsible for content posted by users.

The ASA said many online web forums and comments sections of websites were likely to feature a range of views from across society, with some views being more extreme than others.

Mumsnet said it regularly received more than 25,000 posts per day, an impossible number to monitor, so users were relied upon to report any offensive content which was then taken down.

"We contacted Mumsnet, who reiterated that they did not tolerate any kind of prejudice on the site and this included any kind of gender bias, but users did need to bring this to their attention if action was to be taken," the ASA said.

"We considered that whilst some users of the website had made negative comments about men in its forums, it was misleading of Fathers4Justice to imply through this ad that Mumsnet themselves had made or endorsed those comments.

"We therefore concluded that the ad breached the code. We asked Fathers4Justice not to imply that forum postings on Mumsnet's website indicated endorsement or support from the website itself."

• To contact the MediaGuardian news desk email editor@mediatheguardian.com or phone 020 3353 3857. For all other inquiries please call the main Guardian switchboard on 020 3353 2000. If you are writing a comment for publication, please mark clearly "for publication".

• To get the latest media news to your desktop or mobile, follow MediaGuardian on Twitter and Facebook.