Perverts are swapping sick child porn through the same file-sharing software teenagers use to download music.
The NSPCC says paedophiles have moved to "peer to peer" networks where they can effectively evade police.
It follows the success of Operation Ore which cracked down on vile websites and snared perverts such as actor and comedian Chris Langham.
But it is much harder to crack peer to peer sites. An officer on the Met's paedophile unit said: "No one knows they exist until you are invited on to them. The size of the community or even how many kids are being traded is unclear because they are very difficult to infiltrate."