What floor prep is required before tiling?

[barneyspal]

I want to lay a tile floor in the kitchen. The flat is quite an old building and originally had floor boards throughout but the boards in the kitchen have been replaced with a chipboard floor. Not sure how thick the board is but it is flush with floorboard in adjoining room so I would guess 18-25mm thick. The floor is solid and not bouncy. My question is this: Can I tile directly onto this surface without having to put another covering of ply over the floor? I have read the article on floor prep and that seems to imply that you only need to put down ply if overlaying floorboards. I am hoping to hear that tiling directly onto the floor that is there will be fine as in the next room the floor has been laid with laiminate and I do not want to have too much of a difference in the levels of the floors where they will meet at the doorway. Any advice much appreciated.

[tim'll fix it]

either replace the lot with 25mm ply

or overboard with 12mm ply