The umbrella group representing local authority pension funds has urged its members to vote down National Express' annual report over US labour practices at the transport company's lucrative school bus business.
The Local Authority Pension Fund Forum (LAPFF) said it had made the move following what it said were "representations" from drivers of the famous yellow buses that are used to ferry children to and from school in the US.
National Express is the second biggest private operator in the country, with businesses in 30 US states plus two in Canada.
However, the group has been at loggerheads with the Teamsters Union, which has even held demonstrations at the company's UK headquarters in collaboration with Unite.
The LAPFF said it did not want to take sides. However, it called on the company to expand the remit of its safety and environment committee to include responsibility for strategy on management of staff.
It also wants the committee to report to shareholders on that work.
Kieran Quinn, the forum's chairman, said: "It is clear to us that a number of employees in the US operations of National Express feel the company is not sympathetic to their right to independent representation and this is becoming an increasingly contentious issue. We take no view on the rights and wrongs of these claims. However, this situation has persisted for a number of years now and we feel therefore that the company and its investors would benefit from clearer oversight and reporting of employment issues."
The LAPFF, which represents pension funds with more than £115bn in assets, pointed out that it had previously backed the National Express board in its battle against hedge fund Elliott Advisors, when the activist built up a stake of nearly 20 per cent and sought to force the election of new directors. The fund has since halved its stake in the business.
However, now that the battle is over the LAPFF said it felt it had to act in the wake of the discussions with US staff. The organisation said it viewed employee engagement as "crucial" and said it had "spoken with a number of drivers who work within the company's US school bus business about their experiences".
"On the basis of this engagement, the Forum has decided to sign the statement calling for improved oversight and reporting."
"Our workplace rights policy guarantees the rights of all employees, wherever they work. Reflecting the importance of employee relations, and in line with accepted practice, this issue is reported to the whole board. We are confident this remains the correct approach."