* Investor Mining and Tailings Safety Initiative has 77 member firms

* 126 companies yet to commit to best practice

* Church pensions group may vote against chairs at AGMs

LONDON, Jan 24 (Reuters) - A group of investors managing more than $25 trillion said they plan to challenge mining companies that have not yet committed to a tailings dam best-practice standard and may vote against management at upcoming annual meetings.

The Investor Mining and Tailings Safety Initiative was launched in August 2020 in response to the Brumadinho disaster in Brazil where 270 people were killed when a tailings dam collapsed.

So far 77 listed mining companies, including the world's largest BHP and Rio Tinto , have signed up to a tougher code of conduct.

Tailings dams are embankments constructed near mines to store waste in a liquid or solid form.

But 126 companies, many of them smaller, have yet to commit to the standard, leaving their shareholders, insurers, and banks facing greater risk, the investor group's chair Adam Matthews told Reuters.

The initiative names China's Zijin Mining Group Co Ltd , which has also mining interests overseas and has previously run into trouble with tailings dams, Saudi Arabia's state-owned miner Ma'aden, Grupo Mexico and its subsidiary Southern Copper Corporation, among those companies not conforming with the standard.

Zijin, Ma'aden, Grupo Mexico and Southern Copper did not immediately respond to a request for comment.

"It's still concerning that you've got over 100 companies that ... haven't been responsive to the engagement we've been doing," said Matthews, who is also Chief Responsible Investment Officer for the Church of England Pensions Board, a member of the investor group.

The initiative said it wants commitments from laggard companies within the next year, and clarity on when they would implement the standard.

But Matthews said the Church of England Pensions Board would vote against any companies not conforming or complying with the standard that have annual meetings over the next few months.

"We will be voting against the chair of the company ... and we'll also consider filing a shareholder resolution as well," he said. He also said other members of the initiative could well do likewise but would make their own decision on how to vote.

To date, more than 14,000 active and legacy tailing dam facilities across 77 countries have been collated in a global public registry. (Additional reporting by Daina Beth Solomon. Editing by Jane Merriman)