In its annual National Strategic Assessment, the National Crime Agency (NCA) stated last year that there were 300,000 individuals in the UK who posed “a sexual threat to children.” While 35,000 were being managed by law enforcement, the remainder were active online, on the so-called ‘dark web’, the report elaborated.

This year, the numbers have been revised upwards dramatically. Using a new methodology that the NCA claims better captures the number of sex offenders active online, the organization estimated that anythere between 550,000 and 850,000 offenders may be based in the UK. Of these potential threats, just under 75,000 have been “marked” by law enforcement.

The NCA researchers said that their methodology should be treated with “moderate confidence.” If the upper estimate is correct, however, one in 77 people in the UK could pose a threat to children.

Their offenses fill a broad spectrum, “from downloading and sharing indecent images of children to direct contact abuse,” the report reads.

Yet the true scale of the problem may be even larger. The report notes that the final figure excludes “non-UK offenders targeting UK victims,” and “peer-on-peer” offenders, meaning minors who abuse other minors.

With the majority of offenders lurking online, the NCA cited encryption as the main hurdle faced by law enforcement in tracking down and apprehending child sex criminals. Facebook’s plans to introduce end-to-end encryption of its messages would benefit sex offenders, the NCA claimed, citing the example of David Wilson, a “prolific online offender” charged last year with 96 sexual offenses against boys as young as five.

Wilson created fake identities on Facebook, posing as teenage girls to lure victims into sending lewd images, and blackmailing some of his victims with these images. Evidence turned over by Facebook, including some 250,000 messages, were instrumental in sending Wilson to prison for 25 years.

The NCA warned that encryption would make such prosecutions more difficult, and would “mean other offenders like Wilson would go undetected.”

The anti-encryption theme continued throughout the report, with secure messaging identified as a key tool of organized criminals of all stripes, from human traffickers to fraudsters to drug dealers. Cryptocurrency was also named as a commonly-used money laundering mechanism.

That law enforcement would oppose encryption is no surprise. However, any government attempts to outlaw the practice would face stiff opposition from civil liberties groups, who often cite the importance of encryption to journalists, whistleblowers and activists.