white powdery deposit on clothes after washing?

Hi,
My clothes are covered in a white powdery deposit when they come out of the machine. I've cleaned out the rubber seal (where this substance seems to gather) but as soon as the cycle is complete the seal and my clothes are covered in this substance again.
It's not fluff - could it be limescale or something else?
I've tried running a 90 degree cycle to see if that helps but it doesn't. Any thoughts?
Thanks
M