What is wrong with my Renault Laguna clutch pedal?

I have an 03 Laguna 1.9 dci. The problem is intermitant and is as follows. I will be driving along all ok and suddenly when dipping the clutch the pedal slowly at each press sinks a litrtle more to the floor until there it is almost fully down. If I put my foot under the pedal and pull it upwards it starts to work normaly again and it might be days before the problem reoccurs. It now seems to be happening much more frequently.
Has anybody any idea what might be the cause? I don't know whether it is cable operated or hydraulically.