By Barbara Lewis

BRUSSELS (Reuters) - The European Union will defer a decision on whether to extend approval for weed-killer ingredient glyphosate, used in herbicides including Monsanto's Roundup, following a dispute between EU and U.N. agencies over whether it causes cancer, EU sources said on Monday.

Experts from all 28 member states attended a closed-door meeting on Monday in Brussels, which had been expected to endorse a European Commission proposal to extend authorization of glyphosate for 15 years until 2031.

In an email, the European Commission said the meeting would continue on Tuesday and it could not yet confirm the outcome.

It has said it needs a decision over the coming weeks to prevent a legal vacuum when an existing approval expires at the end of June.

Two EU sources, speaking on condition of anonymity, said they no longer expected a decision at this meeting after France said last week it would oppose extending approval, while EU diplomats said Germany planned to abstain.

"It can go ahead in another group in the coming weeks, months," one source said.

"A decision is very unlikely," another said, referring to this week's talks.

Ahead of the meeting, the European Food Safety Authority (EFSA), which advises EU policymakers, issued an opinion that glyphosate was unlikely to cause cancer.

That conclusion was at odds with the view of the World Health Organization's International Agency for Research on Cancer (IARC). The difference between them has stoked a fierce debate.

Environmental campaigners have called for a ban. In a statement, Greenpeace said governments should oppose renewing the EU license for glyphosate "as long as uncertainty remains over the risks it poses".

A statement on the website of Monsanto, which calls Roundup the flagship of its agro-chemicals business, said it strongly disagrees with IARC's classification of glyphosate as "probably carcinogenic to humans". It has filed a legal challenge in the United States.

The Glyphosate Task Force, which brings together Monsanto and other companies, said there had been a rigorous assessment of consumer safety and it expected approval in the coming weeks.

(Editing by Mark Trevelyan)