All three are still under contract for next season but would be allowed to leave on free transfers if they can find another club.

Reid helped the club win promotion to the Championship two years ago, but played only four times last season and returned to his former club Carlisle United on loan.

Former Welsh international Johnson joined the club last summer after being released by Leicester City but was not involved after the first six games of the season.

Danish striker Christensen was another summer signing who failed to make an impact and has yet to start a game.

A decision has still to be made on the future of Peruvian striker Miguel Mossto, who cost around £750,000 last summer, but failed to adjust to the demands of English football. He is on loan with Peruvian club Coronel Bolognesi until the end of next month.

Goalkeeper Kyle Letheren has been offered a new year deal, but teenage defender Rhys Meynell has been released.