Thanks for the answers.
Made the modification as suggested by SeanB ( new schematic attached) but the circuit still sometimes triggers at the power-on
I've measured the output voltage: unconnected 10,66 V / connected to the circuit 10,48 V
I've noticed also that if I touch with a screwdriver or a lead from the dmm either the Vcc, the GND or the base of the BC548 the circuit triggers