Mumbai, Sep 22 - The Securities and Exchange Board of India (SEBI) Tuesday revised its takeover norms, making it mandatory for holders of global depository receipts (GDR) or American depository shares (ADS) with voting rights to make an open offer if they acquired 15 percent shares of a public company.

Under the existing SEBI takeover code, an entity or individual could not hold more than 15 percent shares in a listed company without making an open offer to acquire an additional 20 percent of that company's shares.

Moreover, the obligation of a mandatory public offer would get triggered only if the ADS and GDRs issued by a company were converted back into shares.

Announcing the changes in the takeover code, SEBI chairman C.B. Bhave also said that there would be no retrospective effect on earlier deals because of this amendment.

In another amendment, SEBI also extended the concept of anchor investor to participate in Indian depository receipt (IDR) issues.

The anchor investor, who cannot be a promoter of the issuer company, can be allocated as much as 30 percent of the portion reserved for qualified institutional buyers (usually 60 percent) in an issue, through a bidding process.