Yes thats chipboard alright and its the older stuff too. Has it got wet anywhere that you can see? Normally outside the sower tray and down the side of the bath, under the basin pedestal and round the bottom of the toilet.
It swells when its wet and goes crumble, the same as weetabix.
It a bit of a mare to change it but it can be done, however if its not got wet then you can overboard it with either a cement backer board or ply. The choice of materil will depend on the amount of deflection in the floors. If there is deflection then you have the option of either overboarding with 15mm minimum ply or removing the chipboard and replacing it with a thicker ply. 18mm upwards.
The backer board option is only really going to work if the floors have no movement. However you can still add them over the ply to provide a more suitable surface for tiling. Of course all that will mean raising the height of the floor somewhat.
