LONDON — The organs of members of marginalized groups detained in Chinese prison camps are being forcefully harvested — sometimes when patients are still alive, an international tribunal sitting in London has concluded.

Some of the more than 1.5 million detainees in Chinese prison camps are being killed for their organs to serve a booming transplant trade that is worth some $1 billion a year, concluded the China Tribunal, an independent body tasked with investigating organ harvesting from prisoners of conscience in the authoritarian state.

“Forced organ harvesting has been committed for years throughout China on a significant scale,” the tribunal concluded in its final judgment Monday. The practice is “of unmatched wickedness — on a death for death basis — with the killings by mass crimes committed in the last century,” it added.

In 2014, state media reported that China would phase out the practice of taking organs from executed prisoners and said it would rely instead on a national organ donation system.

The Chinese Ministry of Foreign Affairs on Tuesday was not immediately available to comment on the tribunal's findings.

In a statement released alongside the final judgment, the tribunal said many of those affected were practitioners of Falun Gong, a spiritual discipline that China banned in the 1990s and has called an “evil cult.” The tribunal added that it was possible that Uighur Muslims — an ethnic minority who are currently being detained in vast numbers in western China — were also being targeted.

The tribunal is chaired by Sir Geoffrey Nice, who worked as a prosecutor at the international tribunal for crimes committed in the former Yugoslavia.

“Falun Gong practitioners have been one — and probably the main — source of organ supply,” the judgment read, while “the concerted persecution and medical testing of the Uyghurs is more recent,” using a different spelling of the minority group's name. It warned, however, that the scale of medical testing of the Uighur Muslims meant they could end up being used as an "organ bank."

The tribunal that delivered its judgment in London was initiated by the International Coalition to End Transplant Abuse in China — a not-for-profit coalition including lawyers, academics, human rights advocates and medical professionals.

Allegations of forced organ harvesting first came to light in 2001, after a boom in transplant activity was registered in China, with wait times becoming unusually short, the statement said. Chinese websites advertised hearts, lungs and kidneys for sale and available to book in advance, suggesting that the victims were killed on demand, it added.

On Monday, the tribunal concluded that there was “numerical evidence” of the “impossibility of there being anything like sufficient ‘eligible donors’ under the recently formed PRC [People’s Republic of China] voluntary donor scheme for that number of transplant operations.”

The tribunal added that witnesses, experts and investigators had told of how Falun Gong practitioners continued to be killed in order for their organs to be extracted. It added that forced organ harvesting was also being performed while victims are still alive, killing the person in the process.

The statement recalled how one witness, Dr. Enver Tohti, told of how as a surgeon in China he had been required to perform organ extractions. Referring to one instance in which he extracted an organ from a living patient, he said: “What I recall is with my scalpel, I tried to cut into his skin, there was blood to be seen. That indicates that the heart was still beating … At the same time, he was trying to resist my insertion, but he was too weak.”

Several survivors of prison camps told the tribunal of how they were subjected to physical examinations including blood tests, X-rays and ultrasounds, the statement said. “Experts report that the only reasonable explanation for these examinations was to ensure that victims’ organs were healthy and fit for transplantation,” it added. A healthy liver, for example, can reportedly be sold for some $160,000, according to the statement.

The tribunal concluded that it was "beyond reasonable doubt" that crimes against humanity had been committed against the Falun Gong and Uighur Muslims but that it could not prove that the killing of the Falun Gong amounted to genocide — because of the tribunal's inability to prove ‘intent’ to commit ‘genocide.’

In a statement accompanying the final judgment, the International Coalition to End Transplant Abuse in China called on the international community to help bring an end to forced organ extraction.

“It is no longer a question of whether organ harvesting in China is happening, that dialogue is well and truly over. We need an urgent response to save these people’s lives,” Susie Hughes, executive director and co-founder of the coalition, said.

Saphora Smith reported from London. Dawn Liu and Ed Flanagan reported from Beijing.