A Seattle-based company has developed a sex chat bot that shocks potential paedophiles once they admit to interest in under-age prostitutes.

Users are convinced with poor grammar and delayed responses that the "person" typing is legitimate. Once the conversation develops enough, the chat bot reveals it is only 15 and asks "are you cool with that?" If the user responds yes, they are met with an instant message explaining under-age prostitution is illegal.

In an extensive report, the BBC's Dave Lee spoke to the companies behind the app as well as a former sex worker. Although the chat bot does not yet have any legal weight, the creators are hopeful it will deter customers by reinforcing a psychological barrier.

"For someone who thinks they're anonymous, who thinks they can go on the internet and buy another human being, it's a big wake-up call," Seattle Against Slavery's Robert Beiser said to Lee. "We work with survivors of trafficking to ask them how a conversation like this would go."

During a test phase, 1,500 people were convinced the chat bot was a real person. In the build up to the reveal, users are even asked for a photo of themselves to proceed.

King County (in Seattle) attorney Val Richey told Lee the internet has led to dramatic growth in the prostitution marketplace. "We found over 130 websites where you can buy sex in the Seattle area alone," Richey said. "One of those websites was averaging 34,000 ads a month last year."

"When we post a fake ad, we'll get 250 responses in the first two hours. There's no way that law enforcement has the capacity to respond to that. The chat bot allows us to connect with and deter all of those buyers online at any time. We've never been able to do that."

Deterring men is just one half of the battle, software is also being used to help active sex workers escape their situation. The software grabs phone numbers from online ads and send out mass messages to prostitutes who may require help.

Because the alert is sent as a text message, it gives the workers a chance to reply when they are safe, as opposed to a phone call. This year, 40 women have been saved because of the software.