P&O has been reported as 'trying to kill the red herring' over poison pills, giving Carnival until Friday next week to put up or shut up, according to the The Daily Telegraph.

Chair Lord Sterling is reported to have written to shareholders detailing why P&O believed its proposed $7bn nil-premium merger with Royal Caribbean was superior to Carnival's conditional offer worth about 450p a share.

However, the letter is reported to make it clear that the door is still open for Carnival to make a better bid. Carnival are firmly standing by their offer, quoted as saying it is already a superior deal.

Last night, P&O Princess shares closed up 3.75 at 398p.