William, who is patron of the Football Association, said in his message: “Incredibly saddened to hear of the passing of Kevin Keegan. A truly remarkable footballer and manager who inspired generations with his talent, passion and love of the game. Above all, he will be remembered as a kind and generous man. Our thoughts are with his family and all who knew him.”
He signed it with his initial, “W”.
Keegan, a star of Liverpool’s 1970s side who went on to become adored on Tyneside as player and manager of Newcastle, had been undergoing treatment for stage four cancer.
A statement from Keegan’s family read: “It is with immense sadness that we announce that Kevin Keegan has passed away at the age of 75.
“Kevin had been battling cancer and was surrounded by his wife and daughters in his final moments.
“A double Ballon d’Or winner, Kevin was a much-loved husband, father and grandfather.
“The family would like to thank Kevin’s incredible medical team for all their support.
“This is a hugely difficult time and they are requesting space and privacy.”