* Principles for Responsible Investment writes to members

* Says Climate Action 100+ not in breach of laws

* Follows exit of several large US members last week

LONDON/BOSTON, Feb 22 (Reuters) - The world's biggest climate investor group has told members its approach does not breach U.S. antitrust and securities law, according to letters seen by Reuters on Thursday, as leaders seek to shore up support days after the shock exit of several large firms.

Climate Action 100+ was rocked last week by the withdrawal of the fund arms of State Street and JPMorgan and bond giant Pimco, while the world's largest asset manager BlackRock scaled back its involvement.

The group aims to facilitate more effective engagement with high-emitting companies to help them transition to a low-carbon economy. Members have been accused of colluding by some U.S. Republican politicians, potentially in breach of the law.

BlackRock and State Street cited the importance of independence when confirming their steps, while JPMorgan said it would rely on its own stewardship capabilities. PIMCO said participation was "no longer aligned" with its approach.

In a letter to members on Wednesday, one of the five investor networks coordinating CA100+, the Principles for Responsible Investment (PRI), urged members to stand firm despite the departure of what it described as "a small number of members", and addressed legal concerns.

"The PRI designs and facilitates initiatives in a way that we believe enables investors to maintain compliance with rules and regulations in key markets, including anti-trust and securities laws in the USA," Chief Executive David Atkin wrote.

"For instance, no initiative ever requires a signatory to vote in a certain way, even for votes or resolutions that fellow investors have put forward or flagged. Collaborative engagements with companies are always investor-led and always voluntary."

Separately, Mindy Lubber, president of investor advocacy group Ceres, another CA100+ investor network, wrote a similar message to members focused on North American companies.

Despite the departures, Climate Action 100+ still has more than 700 investors, including 60 new signatories since June, Lubber wrote. (Reporting by Simon Jessop in London and by Ross Kerber in Boston; Editing by Mark Potter and Cynthia Osterman)