Some paedophiles with images of child abuse will escape prosecution, the head of the National Crime Agency says.
Keith Bristow said the NCA would have to focus on pursuing those who posed most risk but that others would face a "range of interventions".
Some 660 arrests were made during a recent operation targeting people who had accessed child abuse images online.
However, the BBC understands that as many as 20,000-30,000 individuals were identified during that investigation.
Mr Bristow said every one of the images found during Operation Notarise would have to be assessed.
Some of the suspected paedophiles had terabytes - equivalent to 1,000GB - worth of data on their hard drives or storage devices.
NCA director general Mr Bristow said the work to assess the images was "high volume" and had to be done at pace.
"If there are 50,000 people involved in this particularly horrible type of criminality, I don't believe all 50,000 will end up in the criminal justice system," he said at a briefing for journalists.
"It's uncomfortable but we're going work through it in a logical way, target the most risky first."
He said there would be a "range of interventions" which for some of the offenders could fall short of them "standing in a court".
Mr Bristow drew a distinction between "contact abusers" who may have been involved in physical abuse, and those who shared images.
The NCA said it had to focus on those who did the most harm to children.
Society would have to have "deeply uncomfortable conversations" about the scale of child abuse and how to respond to it, Mr Bristow added.