It released the figure, which predates the coronavirus outbreak, with a warning that the lockdown could cause a dramatic increase in online sex offending.
Senior officers said that because of school closures, isolation and social distancing measures, both paedophiles and children are spending more time on the internet.
The NCA warned of a resulting “spike in online child sex offending during the coronavirus crisis” and urged parents and carers to ensure children know how to remain safe.
“With children spending more time online to do school work or occupy themselves while parents and carers are busy, they face an increased threat from offenders who are also online in greater numbers,” a spokesperson said.
“The NCA also knows from online chat that offenders are discussing opportunities to abuse children during the Covid-19 crisis.”
A report published earlier this year by the Internet Watch Foundation (IWF) found that a third of child sex abuse images are originally posted online by children themselves.
Most of the “self-generated images” found were taken by girls aged between 11 and 13, who may originally have sent them voluntarily to peers, or been groomed or coerced by paedophiles – including those posing as children online.
A record of 260,400 web pages were reported to the online watchdog in 2019, of which 132,700 showed children being sexually abused.
The IWF said the estimate that 300,000 paedophiles live in the UK – which was drawn from an NCA strategic assessment – showed a “terrifying escalation of the threat”.
Chief executive Susie Hargreaves OBE said: “Parents may think that, because their child is at home, they are safe but sadly that just not always the case.”
Police are currently arresting more than 500 suspected child sex offenders and safeguarding around 700 children on average every month in the UK.
Chief Constable Simon Bailey, the national police lead for child protection, said: “It is sickening to think that some criminals are looking to exploit the coronavirus crisis to cause harm online.
“Despite the issues that the pandemic will cause for law enforcement, child protection is still a priority and we remain totally committed to keeping our young people safe.”
The NCA said indecent images are so prevalent online that they can be found within three clicks.
Rob Jones, NCA director of threat leadership, said child sexual abuse remained a “priority threat” during the coronavirus outbreak.
“Though we are working around the virus like everyone else, we are continuing to pursue high-risk online offenders to ensure they are arrested and children are safeguarded,” he added.
“The internet has undeniable benefits to society, but it’s also enabled a section of society to commit increasingly horrific crimes against children through grooming, live-streaming and distribution of indecent images.
“Preventing offences occurring is always crucial and now more so than ever when there is masses of online traffic and a possible elevated threat to children.”
Mr Jones repeated calls for the technology industry to do more to prevent the content being uploaded and shared.
The NCA has launched a campaign for online safety at home, including guidance and educational products for children, parents, carers and teachers.
Since the start of the coronavirus lockdown, reports of potential abuse from teachers and social workers has fallen, but reports from children have remained stable.
Since schools closed, the number of child safety concerns reported through the Child Exploitation and Online Protection command (CEOP) website has stayed largely the same.
The Lucy Faithfull Foundation, which runs a confidential helpline for anyone concerned about their own or someone else’s behaviour towards children, urged people to seek advice.
Donald Findlater, director of the Stop It Now! helpline, said: “Our helpline talks to adults concerned about their sexual thoughts, helping them manage these so that children are safe.
“Some people who pose an online or offline risk to children will be struggling with their sexual thoughts or behaviour and need help to manage them. Right now, in this isolation we all experience, it is important that they get help.”
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