Wall crack

[avidfan]

Im going to refit my bathroom and this will involve moving the fittings about.
The bathroom is in the corner of the house and the outside wall will need to be tiled. This wall has a horizontal crack running across it. Over the years I've chopped it out and filled it during decoration. The crack has always returned. It's not a wide, serious crack, but crack it is. What would you guys advice be regarding tiling over this? I've thought about over boarding with 6mm ply to create a barrier between the tiles and the wall movement. Advice gratefully received

 

[vinn]

Dont use the ply option - it wont work.
If possible its best to find the cause of the crack. What do you think it might be?
Photos of the crack and the outside of the wall might help.
FWIW, there's lots of recent posts in the building forum on various kinds of cracks.