The UN's climate science panel acknowledged on Wednesday that a grim prediction on the fate of Himalayan glaciers that featured in a benchmark report on global warming had been "poorly substantiated" and was a lapse in standards.

Charges that the reference was highly inaccurate or overblown have stoked pressure on the Intergovernmental Panel on Climate Change (IPCC), already assailed in a separate affair involving hacked email exchanges.

The new row focuses on a paragraph in the IPCC's Fourth Assessment Report, a 938-page opus whose warning in 2007 that climate change was on the march spurred politicians around the world to vow action.

The paragraph notably declared the probability of glaciers in the Himalayas "disappearing by the year 2035 and perhaps sooner is very high."

The IPCC said in a statement that the paragraph "refers to poorly substantiated rates of recession and date for the disappearance of Himalayan glaciers."

"In drafting the paragraph in question, the clear and well-established standards of evidence, required by the IPCC procedures, were not applied properly," the panel admitted.

It added: "The Chair, Vice Chair and Co Chairs of the IPCC regret the poor application of well-established IPCC procedures in this instance.

"This episode demonstrates that the quality of the assessment depends on absolute adherence to the IPCC standards, including thorough review of 'the quality and validity of each source before incorporating results from the source in an IPCC report'."

The statement noted that the reference was not repeated in an important "synthesis report" of the 2007 assessment, and stressed the IPCC's "strong commitment" to thorough, accurate review of scientific data.

The IPCC co-won the 2007 Nobel Peace Prize for bringing climate change to the world's attention through a reputation for rigour, caution and fact-checking. Under this process, data are peer reviewed by other scientists and are then meant to be double-checked by editors.

In an exceptional move, the lapses came under public attack from four prominent glaciologists and hydrologists in a letter to the prestigious US journal Science.

They said the paragraph's mistakes derived from a report by green group WWF which picked up a news report based on an unpublished study, compounded by the accidental inversion of a date -- 2035 instead of 2350 -- in a Russian paper published in 1996.

"These errors could have been avoided had the norms of scientific publication, including peer review and concentration upon peer-reviewed work, been respected," according to the letter, which Science released on Wednesday, two days ahead of scheduled publication.

One of the letter's authors was Austrian specialist Georg Kaser, who contributed to a different section of the 2007 report.

He told AFP on Monday that the mistake was enormous and that he had notified IPCC colleagues of it months before publication.

Despite the controversy, the IPCC stood by the overall conclusions about glacier loss this century in major mountain ranges, including the Himalayas.

The report concluded that "widespread mass losses from glaciers and reductions in snow cover over recent decades are projected to accelerate throughout the 21st century."

That would reduce "water availability, hydropower potential, and changing seasonality of flows in regions supplied by meltwater from major mountain ranges (e.g. Hindu-Kush, Himalaya, Andes), where more than one-sixth of the world population currently lives," it added.

IPCC chairman Rajendra Pachauri on Tuesday defended the panel's overall work, a position shared by other scientists, who say the core conclusions about climate change are incontrovertible.

"Theoretically, let's say we slipped up on one number, I don't think it takes anything away from the overwhelming scientific evidence of what's happening with the climate of this Earth," Pachauri said.

Skeptics have already attacked the panel over so-called "Climategate," entailing stolen email exchanges among IPCC experts which they say reflected attempts to skew the evidence for global warming.

The row came as the UN panel began the marathon process of drafting its Fifth Assessment Reports, inviting scientists to lead its work.

The reports, due out in 2013 and 2014, will focus on sea level changes, the influence of periodic climate patterns like the monsoon season and El Nino, and forge a more precise picture of the regional effects of climate change.