Riordan scored 20 goals this season

The 23-year-old, who has been top scorer at Easter Road for the past three seasons, is under contract with the Edinburgh side until October.

And the Celtic website says that the Scotland international will not be eligible to play until 1 January 2007.

Riordan is Celtic's third summer signing, following similar moves for Gary Caldwell and Kenny Miller.

Hibs made several attempts to extend Riordan's stay but the player refused to consider new terms.

He rejected a move to Cardiff last summer and Hibs turned down an offer of �400,000 from Rangers in January.

Lokomotiv Moscow also reportedly bid for Riordan, along with German club Kaiserslautern, but the skillful forward was not keen on an overseas move.

It remains to be seen if Hibs manager Tony Mowbray will continue to pick a player who has made such a valuable contribution to the club's recent success.

Mowbray had no problem fielding captain Caldwell after it was announced he was moving to Celtic.

Next season begins on 17 June for Hibs, when they enter the second round of the Intertoto Cup.