A group of international investors managing more than $12 trillion has written to 500 of the world's top companies calling for more information about the treatment of their employees.

More than 100 institutional investors from 11 countries signed a letter sent by the Workforce Disclosure Initiative (WDI) seeking better data on issues such as diversity, workers' rights and health and safety in their supply chains.

The investors, among them Schroders, UBS, Amundi, HSBC Asset Management, Axa Investment Managers, Legal and General IM, Nordea, Rockefeller & Co, and AustralianSuper, gave the companies a deadline of October 22 to respond.

Coordinated by UK-based pressure group ShareAction, the WDI aims to improve the quality of jobs in the operations and supply chains of multinational companies and is funded by UK Aid from the Department for International Development.

"As companies have been reinforcing their disclosures on environmental topics over recent years, we wish to see a similar effort with social factors," Matt Christensen, Global Head of Responsible Investment, AXA IM, said in a statement.

"Companies should disclose data that is material, consistent and comparable enough to truly understand their approach to workforce management in their annual reports."

The letter marks a scaling up of the WDI's efforts after a pilot year during which it engaged with a smaller group of companies.

"Based on the quality of data collected and currently being reported, companies need to move away from only reporting on their policy intentions and good news stories," said Vaidehee Sachdev, senior research officer at ShareAction.

"Investors and civil society want to see evidence that companies are proactively improving workforce practices for the betterment of business, workers and wider society."