Forced organ harvesting has been carried out "for years throughout China" and members of the Falun Gong spiritual group have "probably" been the main victims, according to a panel of lawyers.
A report by the London-based China Tribunal, released to journalists ahead of a summary to be published online on Wednesday, concluded that "forced organ harvesting continues till today".
The panel said it was "unanimously, and sure beyond a reasonable doubt that in China forced organ harvesting from prisoners of conscience has been practised for a substantial period of time involving a very substantial number of victims".
"Falun Gong practitioners have been one and probably the main source of organ supply," it added.
The China Tribunal was set up by campaign group the "International Coalition to End Transplant Abuse in China" to examine the issue.
The panel, headed by British lawyer Geoffrey Nice, warned that the "concerted persecution and medical testing" of China's Muslim Uighurs meant "evidence of forced organ harvesting of this group may emerge in due course".
The report cited "extraordinarily short waiting times" for organs and the number of transplant operations performed as evidence, highlighting the "impossibility of there being anything like sufficient 'eligible donors'."
China has repeatedly denied accusations that it takes organs from prisoners of conscience.
A government spokesman issued a statement before the report's release saying "we hope that the British people will not be misled by rumours".