The latest figure is significantly higher than the previous estimate of 300,000, which was calculated based on the the number of registered sex offenders as well as the amount of people who look at child abuse images online.
Using new methodology which includes those involved in physical abuse, the NCA now estimates that the hidden population of offenders lies somewhere between 550,000 and 850,000 people.
In its annual National Strategic Assessment, the enforcement agency believes its attempts to catch paedophiles would be jeopardised if Facebook follows through with its plan to introduce end-to-end encryption for its messages.
The NCA said this was a mistake, citing the example of David Wilson, from Norfolk, who was jailed for 25 years for 96 sexual offences against 51 boys aged between four and 14.
Fake profiles he had set up on Facebook to groom these boys were used as key pieces of evidence in his successful prosecution, the assessment said.
The report’s authors therefore warned that encryption on the platform “will prevent access to message content and likely mean other offenders like Wilson will go undetected”.
Lynne Owens, the NCA’s director general, also urged social media firms to improve their safety designs.
She said: “While the NCA will continue to lead the fight to cut serious and organised crime, it is imperative that technology and social media companies match this intensity, building in safety by design and closing down all avenues for offenders to exploit their platforms.
“In particular, we must move to a place of zero tolerance for the presence of such material online in order to raise the bar to offending and, most importantly, protect children.”
Her comments come as child abuse is thought to be on the rise, amid increased online activity during the pandemic.
In the final nine months of last year, the police arrested 320 people – one of whom was the deputy head teacher of a primary school – in a campaign against child sex offenders.