More than half a million men in the UK may have viewed child sex abuse images on the internet, a new report claims.
The figure - calculated by the NSPCC based on previous research - is far higher than previous estimates.
It sparked calls for the issue to be treated as a "social emergency".
A report from the charity cited a German survey which found that 2.4% of men reported having consumed child sexual abuse images.
The NSPCC study said: "If we were to apply the same level of self-report data to the male UK population... and applying standard statistical techniques to calculate a confidence interval, this would equate to an estimate that there may be between 450,000 and 590,000 males aged 18-89 in the UK who have at some point viewed and used child sexual abuse images."
It would represent a major leap compared to previous assessments of the scale of the problem in Britain.
In 2013 it was suggested that around 50,000 UK-based individuals were involved in downloading and sharing indecent images of children.
Last month police chiefs said they fear the number may have risen significantly since then, with one report putting it at up to 100,000.
Peter Wanless, chief executive of the NSPCC, said: "The sheer numbers of people viewing child sexual abuse images online must be addressed as a social emergency.
"It is two years since Government made it a national priority to rid the internet of these vile crimes against children, but today's report reveals how horrifyingly prolific the problem remains."
The charity's report said that in the past five years the number of "obscene publications" offences recorded by police has more than doubled, although the category includes a variety of crimes and is not focused exclusively on children.
In April it emerged that record numbers of web pages containing child sexual abuse images are being detected, with 68,092 URLs identified and taken down by a watchdog last year - a rise of 118% on 2014.
The NSPCC called for internet firms operating in the UK to sign up to a minimum set of standards, and for the Government to produce an annual report on the identification and removal of child abuse images accessed from within this country.
Mr Wanless said the charity recognises that progress has been made.
"For example, the work of the National Crime Agency and the police has safeguarded record numbers of victims and arrested hundreds of suspects in the UK," he said.
"And industry is working with partners such as the IWF (Internet Watch Foundation) to identify and remove child sexual abuse images. But these efforts alone will not solve the problem.
"That's why today we are calling for a robust action plan to cut off the supply of child sexual abuse images in circulation, and deter adults from seeking out child abuse online.
"We should be long past the point when there are dark corners of the internet where these terrible crimes against children are hosted for the pleasure of paedophiles."
Meanwhile, a separate study by Middlesex University London found that around six in 10 police officers had investigated some form of online child sexual abuse.
Professor Julia Davidson, from the university, said: "It is clear from our own research... that online child sexual abuse is an international problem that requires global solutions.
"Police officers are responding to online child sexual abuse cases on a weekly basis."
Simon Bailey, National Police Chiefs' Council lead for child protection, said: "While we may never know the full extent of people viewing child sexual abuse images online, the estimate provided by the NSPCC highlights the potentially shocking scale of what we are now dealing with.
"Tackling online child sexual exploitation is a national priority for law enforcement and we are pursuing more offenders and safeguarding more children than ever before.
"This activity has led to an average of around 450 children protected and 375 offenders' arrested every month, and these figures continue to grow."
He added: "We agree with the NSPCC that the police alone cannot stop the demand for child abuse images and more needs to be done to prevent abuse in the first place."