Morningstar, the financial markets research firm, is under investigation from the Securities and Exchange Commission, the US regulator, just days after it was subpoenaed by Eliot Spitzer, the New York attorney general, over a potential conflict of interest.

The SEC began its investigation into two of the US-based data provider's divisions, Morningstar Associates, which advises on pension plans, and Morningstar Investment Services, an investment advisor, on Friday, it has emerged.