Shenton, 23, has made more than 135 appearances for the Tigers since his debut in 2004, and has earned eight caps for England.
The Tigers have now confirmed that they will not be retaining the out of contract star and he now looks likely to move across the Pennines to Saints, who today announced the appointment of Royce Simmons as their new head coach.
Castleford head coach Terry Matterson said: “We would have liked to have kept Michael and made him a big offer to stay. However he has made a decision to move on which we respect and we are very grateful for his contribution in what will have been seven seasons with the club.
“Michael is also a real professional and I know that he’ll be as keen as anyone over the remaining fixtures to help us win us a place in the top-eight.”