A classified ads website is being used to exploit sex workers and trafficked women, a BBC investigation has found.
Panorama has evidence that shows pimps are using Vivastreet to advertise multiple women under their control.
The programme found that one mobile phone number was being used for 78 ads on the site - which suggests the women might be controlled by a single person.
Vivastreet says it is committed to eradicating any potential exploitation.
The website, where people also trade cars and household goods, says providing a safe platform is its top priority.
The site has an established section for adult listings, and prostitution is legal in the UK as long as the sex workers are acting of their own free will.
However, acting as a pimp - exerting control over sex workers for financial gain - is not legal.
Panorama analysed data from the site and found that some of the adverts for sex contained the warning signs of sexual exploitation.
One such sign is when multiple women are advertised using the same phone number, and the programme found hundreds of examples of this.
Many of the adverts also used the same language, including identical phrases and spelling mistakes, which again suggests they were placed by the same person.
Diana Johnson, chair of the All Party Parliamentary Group on Commercial Sexual Exploitation, told the programme it was time to get tougher with sites advertising sex for sale.
"Currently in the UK we are a pimp's paradise," said the Labour MP. "Our laws in this country are out of date. I think we need to look at those laws again and actually make them fit for purpose."
There have been numerous prosecutions of pimping and sex trafficking involving people who have used Vivastreet to advertise the women they are exploiting.
In one case in the north-west of England, a man - Catalin Mihailescu - spent £25,000 placing adverts on Vivastreet in 2017. The website even gave him his own personal account manager.
Det Sgt Stuart Peall, who runs the exploitation team at Lancashire Police, told Panorama that the cases he has worked on all involve Vivastreet.
"Every single job is Vivastreet, they advertise over Vivastreet," he said. "It is very common knowledge that if you need sexual services, Vivastreet is the place that you will find it. You can arrange what you want the girl to look like - it's like a takeaway menu."
Panorama investigated a man in Northern Ireland who was running at least three brothels in Belfast and advertising the women on Vivastreet.
The programme tracked him as he picked up three different women from the airport and transported them to his brothels.
Panorama's evidence suggested he was trafficking women and that they were also being advertised on Vivastreet.
The website says it proactively reports suspicious content to police forces around the country and assists with prosecutions.
"We take any allegations of exploitation extremely seriously and have a range of stringent measures in place to detect criminality, remove bad actors and block users identified as high risk," says Vivastreet.
"This year alone, more than 29,000 ads have been rejected outright and not allowed on the site."
The company says its staff have been trained so they are alert to the signs of exploitation and know what steps to take if they suspect trafficking.
It says it is working with the National Crime Agency and the Home Office to develop an industry-wide approach to preventing exploitation and online trafficking.