Vice Squad cops were said to have nabbed three women and a man from Manchester after they tried to sell six girls aged between 14 and 23 at a five-star hotel in West London.
The gang was secretly filmed while making the offer during which they allegedly boasted about peddling even younger, prepubescent girls.
"This is about as sick as it gets. These are among the youngest girls we've found being offered for sex," the Sun quoted a senior insider as saying.
"This gang was extremely greedy - it was all about the money. Sex with a virgin aged 12 or under cost anything up to 50,000 pounds. Our inquiries continue and we expect more arrests," the insider added.
A spokesperson for UK's National Society for the Prevention of Cruelty to Children (NSPCC) said: "Unscrupulous traffickers are controlling and terrorising vulnerable children within the UK, forcing some into the sex industry and abusing and exploiting many others.
"When the police and other agencies find these children, they need to provide immediate protection and support to help them overcome their ordeal.
"We urge anyone who fears a child is being exploited to call the police, children's services or the NSPCC."