Company has come under sustained criticism for failing to tackle far-right views on its network
Twitter has apologised after letting people advertise specifically towards neo-Nazis and homophobes.
The company gave people the option of targeting their posts at people who hold a variety of far-right views.
It comes as it faces sustained criticism for allowing extremist conversations and users in its social network.
As with other such sites, Twitter gives advertisers the ability to choose who they target ads towards, from a variety of different categories. They focus on their interests as well as their demographics.
An investigation by the BBC found discriminatory terms such as "transphobic", "white supremacists" and "anti-gay" were not being excluded during the ad set-up process.
"Neo-Nazi", which also slipped through the net, could reach an estimated audience of between 67,000 and 81,000 people in the UK, the report claims.
Meanwhile, 92,900 to 114,000 accounts were predicted for the keywords "islamophobes", "islamaphobia", "islamophobic" and "#islamophobic".
Despite Twitter's website saying adverts can be reviewed before running in campaigns, it is claimed generic test adverts using offensive terms were approved.
"They are submitted for approval on an automatic basis, based on an account's advertising status, its historical use of Twitter and other evolving factors," it states.
The report said it was possible to target vulnerable groups as well, such as the keywords "anorexic", "bulimic", "anorexia" and "bulimia" at 13 to 24-year-olds.
Twitter responded saying it does ban certain sensitive words from its ad tool, but the particular examples were an "error" and it has now fixed the issue.
"Twitter has specific policies related to keyword targeting, which exist to protect the public conversation," a spokeswoman said.
"Preventative measures include banning certain sensitive or discriminatory terms, which we update on a continuous basis.
"In this instance, some of these terms were permitted for targeting purposes. This was an error.
"We're very sorry this happened and as soon as we were made aware of the issue we rectified it.
"We continue to enforce our ads policies, including restricting the promotion of content in a wide range of areas, including inappropriate content targeting minors."
Additional reporting by agencies