British startup Nadgerz Inc has created a sex toy that allows the testicles to be used for penetrative sex.

Billed as "the world's first ball-dildo", the Balldo cradles the user's testicles within a silicone container that can be inserted into a partner.

"A new era in sex has arrived," reads the product text.

"We've created the world's first sex toy that will allow you to penetrate your partner with your balls and unleash pleasure that you've never experienced before."

The Balldo is designed around the understanding that the scrotum contains approximately 6,000 nerve endings, with a distribution similar to in a woman's vulva.

According to Nadgerz Inc, this means it can be used to deliver a different type of orgasm that they dub a "ballgasm".

To use the Balldo, the wearer first applies lubricant to cleanly shaved balls, then stretches the Balldo so the ball sack sits comfortably inside it.

Two spacer rings are then pulled up over the Balldo, creating a similar level of rigidity to a dildo.

The tip of the Balldo has the same shape and proportions as a penis, to allow for comfortable penetration, but the sides are open to allow the balls to be stimulated.

"Our patented inner core keeps the side legs firm for thrusting but still allows upper ring to stretch wide open for ease of putting on," reads the description.

Nadgerz Inc co-founder and serial entrepreneur Jerry Davies came up with the idea for the Balldo in a "eureka moment".

Describing the journey behind the project on Starter Story, he recalls "thinking and thinking it over, and realising I was on to something potentially very big".

He and business partner Elizabeth Plasencia spent three years developing the design, testing over 100 prototypes, before launching the design on crowdfunding website Indiegogo.

The company has taken out a series of patents, with the intention of owning the ballsex market. Their aim is for the word balldo to become as ubiquitous as the word dildo.

Nadgerz Inc plans to follow the Balldo with at least two more ball-focused sex toys, in a range of variations.

"It's fair to say we do have the world by the balls" said Davies.

Other recent innovations in the sex toy market include gender-neutral sex toy Enby, which is shaped like a saddle, and Buck Off, the world's first sex toy for transgender men.