BRUSSELS (Reuters) - The head of Europe's food safety watchdog has written to a group of nearly 100 senior scientists strongly rejecting their criticisms in a row about the safety of weed-killer ingredient glyphosate.

The European Food Safety Authority (EFSA), which advises European Union policymakers, issued an opinion in November that glyphosate is unlikely to cause cancer.

That was at odds with a view from the World Health Organization's International Agency for Research on Cancer (IARC), sparked outrage among environmental campaigners and divided the scientific community.

The IARC said in March that glyphosate is "probably carcinogenic to humans" while environmental groups have been calling for a ban on glyphosate.

Ninety-six academics from around the world signed an open letter to European Health Commissioner Vytenis Andriukaitis, dated Nov. 27, urging EU authorities to ignore the European watchdogs's opinion.

"We urge you and the European Commission to disregard the flawed EFSA finding on glyphosate in your formulation of glyphosate health and environmental policy for Europe," the letter said.

It was written by Christopher Portier from the U.S.-based non-governmental organization the Environmental Defense Fund. Portier was also a specialist consulted during the IARC's research on glyphosate.

The letter called for "a transparent, open and credible review of the scientific literature".

EFSA's opinion could lead the 28-member European Union to renew approval for glyphosate, which was brought into use by Monsanto in the 1970s and is used in its top-selling product Roundup and many other herbicides around the world.

In a reply to Portier dated Jan. 13, EFSA Executive Director Bernhard Url described glyphosate as "a keenly debated issue".

"I strongly disagree with your contention that EFSA has not applied open and objective criteria to its assessment," Url wrote in the letter, seen by Reuters.

Url said representatives of EFSA and the IARC would meet early this year to clarify differences of view between the two bodies and that the IARC evaluations "represent a first step".

EU sources said the meeting would probably take place in Brussels in mid-February.

EFSA, based in Parma, Italy, also noted its reply was to Portier and the scientists who signed the letter, not the IARC.

"We should not compare this first screening assessment with the more comprehensive hazard assessment done by authorities such as EFSA, which are designed to support the regulatory process for pesticides in close cooperation with member states in the EU," Url said.

No one at the European Commission or Monsanto was immediately available for comment.

A spokeswoman for IARC told Reuters the Lyon-based agency did not wish to comment at this point.

(Additional reporting by Kate Kelland; editing by Dale Hudson and David Clarke)