Richard Huckle, 30, from Ashford in Kent, admitted an unprecedented number of offences against children aged up to 12 years old between 2006 and 2014.
The freelance photographer bragged about his crimes on the "dark web" and compiled a paedophile manual.
Twenty three children from mainly poor Christian communities in the Malaysian capital of Kuala Lumpur were identified in 91 charges, although it is believed Huckle abused up to 200 young victims.
Huckle took in the families of his victims by posing as a respectable English teacher doing good work abroad.
Details of the abuse can only be reported today because investigators had sought a court order allowing time to make sure the victims were safe from other sexual predators stalking the web.
Practising Christian Huckle first visited Malaysia on a teaching gap year when he was 18 or 19 and went on to groom children while doing voluntary work.
He was arrested as he arrived at Gatwick Airport by National Crime Agency officials on December 19, 2014 as he came home for Christmas, the Press Association reports.
They uncovered numerous indecent pictures and videos Huckle took of himself abusing the girls and boys during his visits to the Far East. In all, more than 20,000 indecent images were found on his computer.
Huckle wrote a paedophile manual called Paedophiles And Poverty: Child Lover Guide as well as a "Pedopoints ledger" in which he detailed rapes and various sex acts.
The manual was on an encrypted space on Huckle's laptop ready for publication on the "dark web" through a hyperlink.
His "Pedopoints" ledger was a private record of the children he had abused. He had made it a game of different types of indecent activities. Under the rules, he could not score for the same activity for the same child in the same week.
The ledger was also on an encrypted part of his laptop and he had written about the Pedopoints on a members-only paedophile site called TLZ (The Love Zone) which has since been closed down.
Huckle was also accused of attempting to crowd-fund his abuse online using Bitcoin on a website called PedoFunding.
In January, Huckle pleaded not guilty to all 91 charges but ahead of his trial in April, he gradually admitted 71 of the offences over the course of five more hearings. At his first plea hearing at the Old Bailey, it took more than an hour to read out all the charges.
According to one charge, Huckle had bragged: "I'd hit the jackpot, a 3yo girl as loyal to me as my dog and nobody seemed to care."
He allegedly forced a three-year-old girl to lie naked with him on a bed holding a handwritten advert, the court heard.
Another charge stated that he published an advert online to sell or distribute indecent photos for his "crowd-funding" campaigns in the autumn of 2014.
At an earlier hearing, Judge Peter Rook QC said: "You have pleaded guilty to 71 counts. As you know these amount to sexual offending of the utmost gravity and as a result I am considering life sentences in your case.
"I am also considering the minimum sentence you must serve. In any event you must expect a very long sentence in this case given that it comes into the category of cases of exceptional gravity. As everyone in this court will know, there is no distinction made between victims here or victims overseas. The cases are regarded as the utmost gravity in both cases."
Twenty two of the offences he pleaded guilty to carry discretionary life sentences and 18 of these are multiple incident counts.
Some 14 of the charges were for rape, five digital penetration, 31 sexual assaults, six grooming offences so children were exposing or touching themselves or other children sexually; 12 counts were for taking very many photographs, one count was for making photographs and one of advertising indecent photographs plus one of arranging or facilitating child sex offences relating to the manual.
Prosecutor Brian O'Neill QC said: "The admitted abuse includes almost every conceivable aggravating feature. Accordingly having considered the pleas offered, tendered and now entered by this defendant, the Crown is of the view they are an adequate reflection of his criminality and the offences to which he has pleaded guilty inform the court sufficiently for sentencing.
"The public interest does not require a trial on the outstanding counts on the indictment and in due course the conclusion of the sentencing process we will invite the court that the outstanding 20 counts lie on the file."
Huckle appeared before the Old Bailey for his sentencing on Wednesday which is expected to conclude on Friday.