The 27-year-old utility player has battled cancer in the past but a recent scan showed it had returned.
Cooksey joined Grays in the summer after stints with Crawley Town, Oldham, Rochdale and Boston United.
At Crawley, he was named Press Player of the Year in 2003, while at Rochdale he played sixty-four games, scoring eight goals and was a firm favourite among the fans.
This season he has played nineteen games for Grays scoring one goal. He has been used in a variety of positions but has always been at his best in the thick of it at midfield, working from box-to-box.
After another barnstorming performance in the victory against Woking in November, directors of the Surrey side turned to the press and said: "If only we had an Ernie Cooksey."
A Grays spokesman said: "The chairman, board and staff of Grays Athletic wish Ernie all the very best and hope he has a speedy and full recovery. I am sure that the fans of the club will join us in giving Ernie our full support."
Source: http://www.nonleaguedaily.com/