The 29-year-old Welshman was told on Wednesday night that the board's doctors were not satisfied with the results of his annual medical and refused him permission to fight on the Frank Warren bill.
Regan and his manager and trainer, Dai Gardiner, asked for a second opinion, but after a further check-up yesterday Regan was told he could not fight.
It is a terrible blow for Regan, who has not fought since he beat Daniel Jimenez to take the title in Cardiff in April 1996. Since then, he has struggled against a succession of illnesses and also late withdrawals of opponents.
Gardiner said: "He's gutted, naturally. He's finally clear of the effects of the virus that hit him and now this happens. It's heartbreaking."
The BBBC have not yet revealed the reason for failing Regan, but it seems that his career could be over without once defending his world crown.