Hull FC have announced that 22-year-old half-back Jamie Ellis has been released from the club with immediate effect.
Ellis joined the Black and Whites on a two-year contract at the start of the season but has failed to establish himself in the first team.
After being given permission to talk to other clubs, he signed a two-year deal with Castleford in June that will take effect from from 2013.
And Hull have now said the player has left the KC Stadium early and that he is free to join another club for the rest of the year, with a Championship side expected to be his short-term destination.
Hull director of rugby Shaun McRae said: "We thank Jamie for his contribution this season and wish him well for the future."