International Paper last month stirred up a potential bidding war over DS Smith, making a takeover proposal that valued the UK firm at 5.72 billion pounds ($7.24 billion), or 415 pence per share.

The U.S.-listed paper company has yet to make a firm offer for DS Smith. It, along with rival suitor Mondi, have until April 23 to make a firm offer or walk away.

International Paper said "significant progress has been made" in due diligence, as it lays out detail on the possible combination, including a proposal to establish a European headquarters in London and a secondary share listing on the London stock exchange.

Shares in DS Smith were up 1.3% by 1121 GMT, and hit their highest level since October 2021 at 408.2 pence.

International Paper shares were down 1.2% premarket.

DS Smith had reached an in-principle agreement with its UK-listed rival Mondi, which made an all-share takeover proposal valuing DS Smith at 5.14 billion pounds.

($1 = 0.7901 pounds)

(Reporting by Yadarisa Shabong in Bengaluru; Editing by Shilpi Majumdar)