But the California Public Employees Retirement System (Calpers), which holds nearly $760m in GSK stock, stopped short of threatening to sell out.

A GSK spokeswoman told BBC News Online the company had been working with Calpers for some months about the issue and understood no stock would be sold.

"Calpers are very supportive of GSK's past work and commended us for our past efforts," she said.

She confirmed that she had seen a draft of the letter.

Dear Jean

Calpers board on Monday voted unanimously to send a letter to GSK chief executive Jean-Paul Garnier to investigate the voluntary licensing of generic versions of the company's Aids drugs.

"We believe this should include continued evaluation of the means by which GSK can offer the lowest possible price on its AIDS therapies without long term harm to the company and in recognition of the balance you must strike between economics and reputational costs," the letter said.

The letter also warned that GSK's "corporate behaviour" could damage its reputation and the value of its stock.

"We are the leader in this field but we recognise this is a serious issue and that people are concerned," the GSK spokeswoman said.

Calpers asked GSK to report back in three months.