Regulators are carrying out a sweeping investigation of conflicts of interest at the US's largest accounting firms, asking whether consulting and other non-audit services they sell undermine their ability to conduct independent reviews of public companies’ financials, according to people familiar with the matter.

The Securities and Exchange Commission probe highlights the agency’s new focus on financial-market gatekeepers such as accountants, bankers and lawyers. These firms help companies raise capital and communicate with shareholders, but also have duties under federal investor-protection laws.