ShareAction ranked Threadneedle as the most responsible in a survey of 33 UK asset managers who invest a combined 13.8 trillion pounds on behalf of pension funds, charities and universities around the world.

Only 42 percent of those surveyed disclosed policies on how they include environmental and social factors in their investment decisions, though this is up from 34 percent in 2010, when ShareAction last conducted the survey.

"A wide range of big names in asset management ... still refuse to be transparent about how they invest clients' money," ShareAction Chief Executive Catherine Howarth said in a statement.

All the surveyed asset managers have signed up to the UK Stewardship Code, first published in 2010 in the wake of the global financial crisis, and 31 out of 33 have signed up to the U.N. Principles for Responsible Investing.

(Reporting by Carolyn Cohn, editing by David Evans)