Spark plug for high frequency ignition system

A spark plug (10) for a high frequency ignition system comprises a centre electrode (24) having a front end with a firing tip (28); a tubular insulator member (20) having a front end (26) and an axial bore (22) accommodating the centre electrode (24), the electrode firing tip (28) protruding beyond the insulator front end. A counter electrode (16) comprises a shell portion (12) receiving said tubular insulator member (20). The tubular insulator member (20) comprises a core (30) made from a dielectric material exhibiting a first dielectric constant and a peripheral layer (32) in a material exhibiting a greater dielectric constant that faces the shell (12) portion of the counter electrode.