The 23-year-old was identified as a priority transfer target by Rangers manager Steven Gerrard but the Ibrox club may now have to look elsewhere in pursuit of central defensive reinforcements.
Millwall manager Neil Harris had publicly declared that Cooper was simply not for sale after Rangers’ initial bid for the former Reading man was turned down.
Rangers felt their second offer represented a fair valuation of Cooper, who is under contract at Millwall until 2020, but the English Championship outfit informed them yesterday they would not accept it.
Mark Allen, the Rangers director of football, has drawn up a list of alternative targets which is believed to include Blackpool’s £1 million-rated Curtis Tilt.
Ipswich Town have already been unsuccessful with an offer of around £600,000 for the 26-year-old this summer.
Gerrard, meanwhile, continued his overhaul of the Rangers first team squad yesterday as Mexican striker Eduardo Herrera returned to his homeland on a season-long loan deal with Santos Laguna.
The 29-year-old was signed by Rangers from Pumas for around £1.5 million a year ago by former boss Pedro Caixinha.
Herrera struggled to establish himself at Ibrox, making the starting line-up on only two occasions.
He made a further 21 appearances as a substitute, scoring two goals, and has two years left on his contract at Rangers.
He becomes the fifth player to be loaned out by Gerrard this season, joining Carlos Pena (Necaxa), Jason Holt (Fleetwood Town), Joe Dodoo (Blackpool) and Ryan Hardie (Livingston) in making a season-long temporary move.