The plot thickens at ImClone Systems as the identity and intentions of the mystery bidder for the US biotechnology firm will be revealed in the next couple of days.

ImClone has issued a statement saying that “the large pharma company” that had made an offer to acquire the firm at $70 per share, subject to due diligence (ended on September 29), but not financing, is ready to act. The suitor has informed ImClone that by a minute to midnight this Wednesday (October 1), “they will either make a proposal, that is no longer subject to due diligence, or withdraw”.

ImClone added that “as has been mutually agreed, if they withdraw and negotiations have thus been ended, the name of the company will be disclosed”.

News that the mystery bidder is preparing to some sort of move has put the spotlight on Bristol-Myers Squibb, ImClone’s US marketing for the cancer drug Erbitux (cetuximab), which is looking to acquire the 83% of the firm it does not already own for $62 a share. That sweetened bid, up from a $60 offer, was dismissed by Carl Icahn, the chairman of ImClone, as "absurd" last week and it is not clear, after a bitter exchange of published letters, whether the two firms are talking.

Rumour is rife as to which is “the large pharma company” that is considering an offer. The Wall Street Journal has cited a person involved in the discussions as saying that more than one company had expressed interest in exploring a transaction with ImClone and these include Pfizer and Eli Lilly. Other firms mentioned in dispatches include GlaxoSmithKline, Takeda, Merck & Co, Celgene and Amgen.