By Kate Kelland, Health and Science Correspondent

LONDON, Oct 6 (Reuters) - Officials from the U.S. government's health research agency are to be questioned by a congressional committee about why taxpayers are funding a World Health Organization cancer agency facing criticism over how it classifies carcinogens.

An aide to the U.S. House Committee on Oversight and Government Reform told Reuters that National Institutes of Health officials have agreed to give an in-person briefing to the committee after questions were raised by lawmakers over its grants to the International Agency for Research on Cancer (IARC), a semi-autonomous part of the WHO based in Lyon, France.

The hearing will be in private, with NIH officials answering questions from committee investigators, the aide said.

The committee is working with the NIH to schedule the briefing soon, the aide said, but no date has yet been set.

The briefing comes after the committee's chairman added his voice to growing concerns among some senior U.S. lawmakers about the way IARC reviews and classifies substances.

In recent years IARC has caused controversy over whether such things as coffee, mobile phones, processed meat and the weed killer glyphosate cause cancer.

Its critics, including in industry, say it is sometimes too quick to conclude that substances might cause cancer, causing unnecessary health scares. It defends its methods as scientifically sound.

In a Sept. 26 letter to NIH director Francis Collins, Oversight Committee Chairman Jason Chaffetz describes IARC as having "a record of controversy, retractions, and inconsistencies" and asks why the NIH, which has a $33 billion annual budget, continues to fund it.

"IARC's standards and determinations for classifying substances as carcinogenic, and therefore cancer-causing, appear inconsistent with other scientific research, and have generated much controversy and alarm," Chaffetz wrote.

The NIH confirmed in an email to Reuters that it had received Chaffetz's letter and "will respond directly to the committee".

The WHO referred Reuters to IARC for comment. A spokeswoman for IARC told Reuters that Chaffetz's letter contained "misconceptions" which IARC's director, Chris Wild, has sought to address in a letter of his own to the NIH director.

Wild's letter, dated Oct. 5 and copied via email to Reuters on Thursday, rejects Chaffetz's criticisms and says IARC's classifications, known as "monographs", are "widely respected for their scientific rigor, standardized and transparent process and ... freedom from conflicts of interest".

Wild also defends IARC's evaluation of coffee and disputed Chaffetz's description of it as a "retraction". IARC's previous assessment of coffee as "possibly carcinogenic" was updated in June this year, when IARC said it had found "no conclusive evidence for a carcinogenic effect".

"The (coffee) report in 2016 was not a 'retraction' but a re-evaluation based on an additional 25 years of scientific evidence," Wild said.

FULL DISCLOSURE

Chaffetz, however, asks the NIH to detail its standards for awarding grants and the vetting and oversight of grantees. It also asks for full disclosure of NIH funds to IARC or money spent in relation to IARC's activities.

Questions over grants awarded by NIH to IARC could put a significant portion of IARC's funding at risk.

IARC's resources are relatively modest. Its 2014 revenue was about 30 million euros ($33 million).

In his letter, Chaffetz's cites the NIH's grant database as showing that it has given IARC more than $1.2 million so far this year. The database also shows that since 1992, NIH grants to IARC have totaled some $40 million.

The American Chemistry Council also joined those voicing concern, issuing a statement following Chaffetz's letter accusing IARC of "a long history of passing judgment on substances through a fundamentally-flawed process that yields questionable results".

"We welcome the interest of the House Committee ... and hope it will shed light on the close and somewhat opaque relationship between IARC and NIH, including the use of taxpayer dollars and resources to support IARC's activities," it said.

IARC is also in dispute with the European Food Safety Authority (EFSA) and United Nations and United States regulators over glyphosate, a widely-used weedkiller developed by Monsanto.

IARC says glyphosate is "probably carcinogenic", while EFSA and several other regulators say it isn't.

This dispute prompted Robert Aderholt, chairman of the U.S. congressional Appropriations Subcommittee on Agriculture, to write in June to NIH's Collins questioning funding of IARC.

In that letter, Aderholt says IARC's conclusions "appear to be the result of a significantly flawed process" and adds that "some in academia have raised questions about the quality of the science and the transparency of the process".

The glyphosate dispute also held up a decision on whether to relicense the product for use in Europe. ($1 = 0.8941 euros)

(Reporting by Kate Kelland; editing by Giles Elgood and Peter Graff)