Warner, who is currently Millwall's Player of the Year, has been receiving rave reviews for his performances over the course of the season and is rated by many as one of the best keepers outside of the top flight.
City manager Kevin Keegan is desperate for a goalie to challenge veteran David Seaman for the number one spot and sees 'Denzil', as he is affectionately known, as the ideal candidate for the position.
Warner has spent the last four seasons at The Den after signing on a free transfer from Liverpool.