According to the ASA, the advert on the Jes-Extender website stated: "Clinically tested with excellent results, the Jes-Extender is a pain free treatment to enlarge your penis comfortably and easily."
It went on to tell potential buyers that the unique "traction method" could "grow your penis by up to 24% from its original size".
The ad also told viewers their penis would grow in size by 1.1 inches over four months, adding: "You can expect a weekly average growth of 0.07 inches."
The ASA investigated the advert after a Jes-Extender customer contacted them to complain that the stats were misleading.
On Wednesday morning, the watchdog released a statement saying the ad had been banned from appearing in its current form.
"The ASA noted that the ad claimed that the Jes-Extender could enlarge penis length by almost a quarter of its previous size," the ASA website reads.
"Given the nature of the claim we required Jes-Extender to submit full studies to assess whether the claim could be substantiated. We acknowledged the materials provided by Jes-Extender which included promotional materials, background information, trial summaries and testimonials.
"However, we did not consider these to be sufficient evidence alone to support the claims.
"Because we had not seen sufficient evidence to support the claims that Jes-Extender could enlarge penile length, we concluded the claims were misleading and had not been substantiated."
The advert no longer appears on the Jes-Extender website.