Britain's biggest pension fund investor has warned that it might vote against directors' appointments if companies did not bring more women and members of ethnic minorities into boardrooms.
Sacha Sadan, director of corporate governance at Legal & General Investment Management (LGIM), said a vote against the chairman of a nominations committee might be considered if directors failed to explain properly why they had overwhelmingly male boardrooms.
Supporters of the campaign for diversity argue that having boards with greater female representation and a mix of ages and skills could help companies better identify markets they have not fully exploited.
So far this year, Mr Sadan said his company had met with 250 companies on this issue.
"If a company does nothing for three years, then we would ask why is that the case," he said. "And we wouldn't be alone on that."
He pointed to several companies he saw as models in their commitment to diverse boards, including Imperial Tobacco, which has a female CEO and board members with experience in the manufacturing, legal, and energy fields, and P&G, where 46pc of staff hired in 2010 were women.