Hundreds of cases involving suspected paedophiles remain unresolved more than two years after evidence was first passed to UK authorities.
Figures obtained by the Press Association show that more than 200 suspects are still being investigated after information was first passed to the Child Exploitation Online Protection Centre (Ceop) by Canadian police in July 2012.
Among the 21 UK forces that were able to provide a detailed breakdown of how the Canadian cases - which came out of an international probe dubbed Operation Spade - had progressed, a total of 271 are still ongoing.
The figures also showed that from a total of 724 referrals made to the forces concerned, 34 people had been charged and five had accepted cautions.
"Investigators in Toronto gave 2,345 pieces of intelligence linked to accessing child abuse images to CEOP, but it was only in November 2013 that the information was finally passed on to police, when CEOP was taken in to the newly-created National Crime Agency."
Controversy was sparked when it emerged that the tip-offs included information about disgraced Cambridgeshire medic Myles Bradbury and teachers Martin Goldberg and Gareth Williams, who both secretly filmed children.
Goldberg, who worked at Thorpe Hall School in Southend, was found to have hundreds of images of pupils on his computer when he was discovered dead at his Essex home; while Williams, from Cardiff, is now serving a five-year jail term.
Jon Brown, NSPCC lead for tackling sexual abuse, said: "It's vital any evidence of someone viewing or making child abuse imagery is urgently followed up by forces.
"There are clear links between accessing this material and contact offending so some offenders who make and view this abusive imagery pose a serious risk to children in communities.
"That risk is obviously increased if offenders have access to children through their work, family or accommodation. They could be school teachers, doctors or working with children in some other capacity on a daily basis so they need to be assessed as soon as possible.
"We appreciate these are complex investigations with no easy decisions and that the police are under enormous pressure dealing with the volume of these offences. But it's essential that offenders are quickly identified and authorities take the necessary steps to protect any children they may be in contact with."
Child protection expert Jim Gamble, who was chief executive of Ceop until he resigned in 2010, warned that similar delays could happen again.
He said: "These mistakes correlate directly to the lack of investment that has been made in child protection resources, especially in areas where the internet is involved.
"This government clearly does not understand the issues, they allowed Ceop to wither on the vine.
"There is far too much work for far too few people, and of those far too few people too few have the right specialist skills.
"There are people whose identities are hidden among data streams that are sitting on shelves because police lack the skills to identify and risk assess and the resources to follow it up."
Last week author John Grisham sparked outrage when he suggested some suspects might access child abuse images online by mistake while they were drunk.
Mr Gamble said: "Some people still think that it's somehow different, that looking is not the same as doing, that it doesn't cause the same harm and could be done by accident. That is simply not the case. People who look go on to do."
The former Ceop chief said that the workload for investigators has trebled since his time at the agency.
Earlier this week head of the NCA Keith Bristow apologised for any harm caused to children as a result of the delays in the information being passed to police.
Giving evidence to the Home Affairs Select Committee, he said: "Sitting on data for the period of time between July 2012 and November 2013 that could have led to children being protected or safeguarded, seems to me whether it's systemic or it's down to individuals - and there are certainly some systemic issues that we need to work through - that's not in the spirit of what we stand for and I'm sorry if that's led to harm to children or exposing them to risk because that's not what we stand for.''
Watchdog the Independent Police Complaints Commission is looking at how long Essex police took to act on information about Mr Goldberg, who was eventually visited by officers in September and found dead the following day.
It is also investigating why Ceop took so long to pass on details linked to Bradbury.
National policing lead for child sexual exploitation Chief Constable Simon Bailey said: "Every force in the country will have undertaken a risk assessment of the threat posed to children by referrals. They will then have decided on a course of action based on an assessment of the overall threat."
He said that forces are dealing with an "unprecedented" number of reports of sexual abuse, many historical, and that victims have more confidence in police to deliver justice.
A Home Office spokesman said: "The NCA is currently leading an unprecedented operation against online child abusers in the UK.
"Ceop was brought into the NCA to ensure child abuse investigators have access to the agency's extensive crime-fighting resources and global expertise, which includes officers in 40 countries around the world. The move has also strengthened Ceop by ensuring investigators have specialist support to draw on, such as the National Cyber Crime Unit.
"We will always ensure police and other crime-fighting agencies have access to the powers and resources they need to tackle child abuse in all its forms."