Up to 850,000 people in the UK pose a sexual threat to children, according to the latest estimates published by the National Crime Agency.
Law enforcement officials have used a new method to calculate the size of the hidden population of offenders, and estimate it to be between 550,000 and 850,000.
This includes paedophiles who download images of child abuse, as well as those involved in direct physical abuse.
The new calculation method means the number is far higher than previous estimates, which focused on the number of registered sex offenders and those using sites on the dark web to look at images of child abuse, put at a minimum of 300,000 people.
In its annual National Strategic Assessment, the NCA said plans to introduce end-to-end encryption for Facebook messages may hamper its ability to catch paedophiles.
In one case, prolific offender David Wilson, a labourer from King’s Lynn, Norfolk, used fake identities on social media to pose as teenage girls and groom and blackmail boys to send him abuse images.
Information from Facebook and thousands of messages were key to his successful prosecution for 96 sexual offences against 51 boys aged four to 14, for which he was jailed for 25 years.
The NCA assessment said: “Information from Facebook and evidential material containing over 250,000 messages was crucial to Wilson facing justice.
“However, plans for end-to-end encryption will prevent access to message content and likely mean other offenders like Wilson will go undetected.”
NCA director general Lynne Owens called on social media companies to shut down all avenues for criminals to use their platforms.
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.”
Child abuse is one type of crime believed to be on the increase, according to the NCA, exacerbated by the increase in online activity due to lockdowns amid the pandemic.
Referring to the mid-point of the estimated number of offenders, Andy Burrows, Head of Child Safety Online Policy at the NSPCC, said: “This assessment underlines that the scale and complexity of child abuse has never been greater, with 700,000 adults across the UK likely to pose a sexual threat to children.
“This is an urgent challenge and it’s clear we can’t continue with the status quo, where it’s left to law enforcement to investigate child abuse but social networks fail to do enough to proactively prevent and disrupt it from happening in the first place.
“It couldn’t be clearer that Oliver Dowden needs to deliver a more ambitious Online Safety Bill, with success being judged on whether it does everything necessary to prevent inherently avoidable online abuse.”
In March 2020 as restrictions started, the NCA began targeting high-harm child sex offenders, and by the end of the year had identified more than 1,000 suspects, had along with police arrested 320 people, and safeguarded more than 400 children.
Of the 320 arrested, 122 were targeted by the NCA, and included 17 people in positions of trust, including the deputy head of a primary school who was later jailed.
Other forms of crime thought to be rising are bribery and corruption, cyber crime, drug dealing, fraud and money laundering.
Ms Owens said there had been a sharp rise during the pandemic in ransomware attacks, computer software used to block access to a particular system until a ransom is paid.
She said: “The threat (from organised crime) has proved resilient in the face of Covid-19, with offenders increasingly turning to online spaces, using emerging technologies and well-established tools to avoid detection.
“Consequently, we have seen dramatic increases in ransomware attacks, and growing levels of cyber-enabled fraud.”
The NCA also said that there has been a “marked increase” in the sale of software used to remotely access someone else’s computer amid the rise in the number of people working from home.
Currently, the NCA estimates that 69,281 people in the UK are involved in serious and organised crime, although it is believed this estimate may rise when information from local databases is accessed.
A key operation in 2020 followed the successful hack by French authorities of secret messaging service Encrochat, used by organised criminals to hide their activities.
In the UK alone, the information provided led to 1,550 arrests and the seizure of 5.8 tonnes of class A and B drugs, nearly £57 million in cash, 115 firearms and 2,879 rounds of ammunition.
A Facebook spokesman said: “Child exploitation has no place on our platforms and Facebook will continue to lead the industry in developing new ways to prevent, detect and respond to abuse.
“End-to-end encryption is already the leading security technology used by many services to keep people safe from hackers and criminals.
“Its full rollout on our messaging services is a long-term project and we are building strong safety measures into our plans.”
Full encryption will be brought in during 2022 and the company said it is already focusing on stopping abuse in other ways, including using AI to detect harmful behaviour, safety notices in Messenger and a feature to stop adults messaging under-18s.