The Canterbury-Bankstown Bulldogs and Des Hasler part ways, effectively immediately.
30 hours ago
Castleford are pushing for Super League to be expanded to 13 clubs for the 2007 season so they can avoid the drop.
"We have made representations to the RFL and we will speak to the clubs to see if it's a possibility," said Tigers chief executive Richard Wright.
"Rules are rules but there is an element of injustice here. We believe we are an asset to the competition."
However, Wright admits it may be hard to convince clubs to take a cut in their share of television money.
"It would mean watering down the funding, which is always a consideration," he said.
Castleford could also be handed a back-door reprieve if Whitehaven or Batley win next month's National League One Grand Final.
Only Hull KR and Widnes, who meet on Sunday in the qualifying semi-final, are eligible for promotion.
Castleford were relegated from the top flight when they lost 29-17 to fellow strugglers Wakefield on Saturday.
The RFL have plans to move to a franchise system and a 14-team competition by 2009, effectively removing automatic promotion and relegation.