Syrian security forces have killed at least 256 children since the onset of protests against Bashar al-Assad, shooting dead a two-year-old girl to prevent her from "growing into a demonstrator", according to UN investigators.
The most authoritative inquiry into Syria's violence has uncovered "gross human rights violations" amounting to "crimes against humanity".
Geoffrey Robertson QC, a human rights lawyer, said the British Government should respond by urging Syria's referral to the International Criminal Court by the UN Security Council.
"That would be the next logical step short of armed intervention," he added. "It would send a chill up Assad's spine."
The UN commission of inquiry, established by the Human Rights Council in August, disclosed previously unknown atrocities committed by Mr Assad's forces. Children appear to have been singled out for abuse, with some being tortured "to death" in custody.
Others suffered serious sexual violence. One witness told the investigation that he saw a "15-year-old boy being raped in front of his father". Another testified that he witnessed the rape of an 11-year-old boy "by three security services officers".
A male university student told the commission that he was "subjected to sexual violence in detention," adding: "If my father had been present and seen me, I would have had to commit suicide".
The two-year-old girl, perhaps the youngest victim of Syria's turmoil, was shot dead on Aug 13 by a military officer. His justification was that he "did not want her to grow into a demonstrator," says the report.
In all, 256 children had been "killed by state forces as at 9 November". Some were deliberately denied medical treatment after being wounded. One 15-year-old boy was shot in the leg in the city of Homs on Aug 15 "while returning home from the mosque". His "neighbours tried to take him to hospital, but checkpoints by security forces blocked access".
Hospitals have not always been places of safety, with security officers disguising themselves as doctors to torture and abuse patients, according to the report. "The commission saw several children whose mental health was seriously affected by their traumatic experience," it adds.
The UN commission, chaired by Paulo Pinheiro, a Brazilian lawyer and academic, conducted its work in Geneva without the cooperation of Syria's regime. Interviews with 223 victims and witnesses provided the basis of its report. "Torture, sexual violence and ill treatment were inflicted on civilians suspected of sympathy with protests, regardless of their gender or age," said Mr Pinheiro.
Alistair Burt, the Foreign Office minister responsible for the Middle East, said the report "highlights the horrific and shocking actions carried out by the Assad regime against its own civilian population".
Syria has never signed the Rome Statute that established the International Criminal Court in 2002. Consequently, the only legal avenue for Mr Assad's regime to be held accountable would be for the Security Council to ask the ICC's chief prosecutor to investigate the situation.
But Russia or China, Syria's only allies on the Security Council, could veto any such attempt. Mr Robertson said that Britain should try to secure Syria's referral, using the precedent set in February when the Security Council passed Resolution 1970 sending Col Muammar Gaddafi's regime in Libya to the ICC.
"Given the blatancy of the killings, Russia and China may perhaps be persuaded," added Mr Robertson. "Given that there seems to be no other way to prevent Assad from pursuing a course that can only lead to more violence, I think that Russia and China may find themselves supporting the next stage."
Syria's regime has been shunned by the Middle East's leading powers, with the Arab League imposing economic sanctions on Sunday. However, Lebanon broke with the consensus yesterday, declaring that it would not enforce these measures against its neighbour. Lebanon has close economic ties with Syria, exporting goods worth $333m last year, meaning that any sanctions would damage its own economy.
Nonetheless, Walid al-Muallem, the Syrian foreign minister, denounced the Arab League's decision as "economic warfare" against Syria's people. He added that the country's own security forces had been subjected to countless "atrocities" committed by "armed gangs".