The world’s biggest drugmaker may play spoiler, according to unnamed sources cited by The Business Standard. The speculation comes just one day Japan’s Daiichi Sankyo agreed to pay about $4.6 billion for the 35 percent stake held by Ranbaxy ceo Malvinder Singh (see photo) and his family, and lock up another 17 percent with an open offer, the paper notes.

Pfizer, which held talks with the Ranbaxy founders, may offer to buy out the stake held by lenders and other investors, the sources tells the Standard. When contacted, a Ranbaxy spokesperson tells the paper only that “we have a binding agreement with Daiichi Sankyo and it is a final agreement. We will not comment on market speculation.” A Pfizer spokesperson declined to comment to the paper.

Legal experts believe that competing offers for Ranbaxy could occur despite the agreement with Daiichi. Somasekhar Sundaresan, a partner at J. Sagar Associates, tells the Standard that “anyone can make a counter offer for the non-promoter stake within 21 days of the open offer at a comparable price.”

Pfizer, by the way, is battling Ranbaxy in about 18 countries over Lipitor patent rights. In most countries, the patent will expire starting 2011, and Ranbaxy has won favorable verdicts in many countries, the paper notes. Pfizer wasn’t the only one that lost out to Daiichi. Glaxo also held discussions, according to the Standard.