Kiev's Chernobyl museum shows photos of dead emergency workers
Official UN figures predicted up to 9,000 Chernobyl-related cancer deaths.
But Greenpeace says in a report released on Tuesday that recent studies estimate that the actual number of such deaths will be 93,000.
Stressing that there is a problem with diagnosis, it adds that other illnesses could take the toll to 200,000.
"Our problem is that there is no accepted methodology to calculate the numbers of people who might have died from such diseases," Greenpeace campaigner Jan van de Putte told Reuters news agency.
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The explosion and fire at the Chernobyl nuclear plant in April 1986 was the world's worst nuclear accident.
It spread a cloud of radioactive particles across a huge swathe of Europe.
Several million people still live in contaminated areas.
Disputed figures
The UN figure - of between 4,000 and 9,000 extra cancer deaths - came from a report released last October by the UN-led Chernobyl Forum.
HOW MANY DIED? Acute Radiation Sickness (ARS) deaths in 1986: 28 ARS patients who died later: 19 (some from other causes) Others who died during explosion: 2 Child thyroid cancer deaths (1992-2002): 15 (UN figure) Predicted extra cancer deaths: from 4,000 (UN) to 93,000 (Greenpeace) Estimated deaths from non-cancer causes 1990-2004: 107,000 (Greenpeace) Dozens killed in accidents building sarcophagus (according to an engineer)
How the disaster unfolded
Many emergency and recovery workers, the report suggested, had died since 1986 from natural causes which could not be attributed to radiation exposure.
But in its report, Greenpeace suggests there will be 270,000 cases of cancer alone attributable to Chernobyl fallout, and that 93,000 of these will probably be fatal.
Blake Lee-Harwood, campaigns director at Greenpeace, told the BBC that cancer was likely to be the cause of less than half of the final fatalities.
"We're also looking at intestinal problems, heart and circulation problems, respiratory problems, endocrine problems, and particularly effects on the immune system," he told the BBC's World Today programme.
Child victims
Mr Lee-Harwood cited technical reasons for the discrepancy.
Doctor Oxana Lozova, who works at a children's hospital in Rivne district, 300km (190 miles) west of Chernobyl, said many generations appeared to be affected.
"I think the fallout from Chernobyl has affected the immunity of those who were young children at the time of the disaster," she told the BBC's Moscow correspondent, Damian Grammaticas.
"We now have to deal with people who are a lot weaker than their fathers and grandfathers were.
"They're falling ill at an age when they really should still be quite fit."
'Apples and oranges'
The WHO said comparing the Chernobyl Forum and Greenpeace reports was like "comparing apples and oranges" when it spoke to the BBC News website.
A tendency to attribute all health problems to exposure to radiation have led local residents to assume that Chernobyl-related fatalities were much higher
Chernobyl Forum report, September 2005
"The WHO felt it had recourse to the best national and international scientific evidence and studies when it came up with its estimates of [up to] 9,000 excess deaths for the most affected areas. We feel they're very sound."
Mr Hartl rejected accusations of bias toward the nuclear industry in the report.
"We acting as [neither] an apologist nor an attacker of the nuclear industry," he said.
The original report found more than 600,000 people received high levels of exposure, including reactor staff, emergency and recovery personnel and residents of the nearby areas.
A map showing radioactive contamination by Caesium-137 around Chernobyl

A report published by Greenpeace questions an estimate made by the  (IAEA) that 4000 deaths have resulted from the . Greenpeace claims that this estimate is 'a gross simplification of the breadth of human suffering' and that over the last 15 years, 60,000 have died in  because of the accident, and estimates that the total death toll for  and  could be another 140,000.

The figure of 4,000 deaths given by the IAEA report referred to cancer fatalities within a group of 600,000 people, comprising of those who were sent in to clean up after the accident and those relocated after the accident. The Greenpeace report calls this figure 'misleading', saying that the report 'hid the true scale of human impact of Chernobyl'.

Ivan Blokov of Greenpeace said "it is appalling that the IAEA is whitewashing the impacts of the most serious nuclear accident in human history".

The IAEA have not yet responded to the report. 

The  spokesman Gregory Hartl stated that "The Greenpeace report is looking at all of Europe, whereas our report looks at only the most affected areas of the three most affected countries." Furthermore, "The WHO felt it had recourse to the best national and international scientific evidence and studies when it came up with its estimates of up to 9,000 excess deaths for the most affected areas. We feel they're very sound."

The report has been released at a time when the UK government is considering whether new nuclear power stations should be built in the country.