WILMINGTON, Delaware, Nov 5 (Reuters) - The structure of

Novo Nordisk's $10 billion bid for obesity drug

developer Metsera ( MTSR ) may violate provisions of U.S.

antitrust law if the companies consummate the deal without prior

regulatory review, the U.S. Federal Trade Commission said in a

Wednesday letter to the companies' legal teams.

The warning letter comes as Novo and Pfizer ( PFE ) are

engaged in a heated bidding war for Metsera ( MTSR ) with each raising

its bid this week. Pfizer ( PFE ) has argued Novo's bid carries

regulatory risk, while Pfizer ( PFE ) has obtained early clearance.

Both Novo and Metsera ( MTSR ) did not immediately respond to

Reuters' request for comment.

Novo is a major player in the obesity drug segment, which

Pfizer ( PFE ) is hoping to crack by acquiring Metsera ( MTSR ).